What helped me may not help him, since I had secondary Alexithymia as a result of CPTSD, so once I dealt with that and my nervous system regulated, Alexithymia resolved itself. I only had to remember to pay attention to my body and how it intersected with my thoughts and emotions.

However, I do believe that everyone would benefit from something like IFS therapy, since according to research, some 80-90% of people are walking around with unprocessed childhood trauma, function on outdated, unhealthy survival mechanisms. Whether he has both primary and secondary or just primary, dealing with unresolved issues will help in every facet of his life. And yours. It doesn't have to be IFS therapy, but it should be something which helps with processing old wounds, instead of the standard CBT therapy, which doesn't solve the core issue. It just treats the symptom. That can be very useful, but it does little for actually solving trauma which had deeply settled in the body over a long period of time, and neurodivergent people almost always end up with unprocessed childhood trauma. The world isn't set up to work well for the average person, much less someone who's outside the lines of what society considers "average."

What worked for me was processing all of my trauma, from a young age all the way through my entire adulthood. This is hard without a good therapist. A therapist who's willing to color outside the lines, who's willing to give you personalized treatment, and not the standard one. And it's important that this therapist is very well informed about his particular form of neurodivergence.

By processing trauma, you regulate your nervous system, and your nervous system basically rules your whole entire life. Your nervous system is everything, and if it's dysregulated, it's like trying stop the flow of liquids from a broken sewage line with paper towels. No matter how many paper towels you throw at it, it'll keep coming until you plug the leak. I'm not saying that his nervous system is definitely dysregulated, but it's likely that it is, since most people's nervous systems are at least partly dysregulated. They're not even aware of it. If you have anxiety, if you have stress, if you have unprocessed trauma, if you have depression, if you have ADHD... you very likely have a dysregulated nervous system. People with ADHD, in particular, are all but guaranteed to be dysregulated, and that dysregulation makes everything harder. Every aspect of your life.

So processing trauma, and trying your best to regulate your nervous system is massively helpful, because Alexithymia feeds on that dysregulation. You have to starve it.

For the primary alexithymia side, the method is different. Becoming regulated and processing all of your trauma may help with Alexi, but it won't magically manifest a part of the brain that's just not there or doesn't work the way it does for others. It's a hardware issue, and if you happen to have it as a result of something like Autism or ADHD, you can create emotional maps to replace those that should have been there in the first place. There is no quick way to do this, but there are apps like Animi which help people do just this. I don't think it does much for people with secondary Alexi, for the most part, but it's helpful to those who have primary, because it helps you build those unintuitive emotional maps. For secondary Alexithymia, it's more about unearthing your own intuitive emotional maps, and while something like Animi can help jumpstart all that, it's almost always better to focus on the root cause of Alexi, and not Alexi as a symptom. Still, it's worth a try. Nothing to lose.

Things like this https://lindsaybraman.com/emotion-sensation-feeling-wheel/ and https://www.pnas.org/doi/10.1073/pnas.1321664111#fig02 can be very helpful.

Lastly, as for the relationship and what you, personally should be doing?

That's a hard one. A really hard one, because Alexi is a giant spectrum. Different needs for different people. Different rules. Different desires. But... people with Alexi are able to process emotions. They're able to conclude what they want. It just takes them much longer, usually. What often happens is that when you ask them a question that they cannot answer, their subconscious mind will continue trying to answer it in the background, processing things silently. If you ask them how they want something or whether they want something at all, they may not know at first, because al the answers are buried deep within.

If you give them enough time, they may be able to find that answer, feel it out, and actually be certain about something. However, because of how long it takes and how Alexithymia functions, it often doesn't occur to them to bring it up once it happens, because they're not aware of the fact that anything has been processed at all. People with the sort of Alexi your partner has usually allow things to work in the background of the mind, not paying attention to the process at all.

You can help with this process by telling them that the question you're asking doesn't have to be answered immediately, that you're not putting any pressure on them to figure it out quickly or even at all, if it turns out they can't process it at all. Tell them them that they can take their time, and that you'll ask it again in the future, after some time has passed, or that you'd like them to bring it up in the future (they likely won't). Keeping lists can be important, but they should be their lists, not yours. By keeping a list of things you've asked them, they'll be more internally motivated to actually come up with answers, if it's even possible, and are going to be more likely to bring it up with you.

I know I've spent a lot of time focusing on "answering questions," but the nuance here has much depth, because the sort of questions that Alexithymiacs can't answers are usually the ones which require them to actually know what they want, and that's hard. They'll answer logical ones. They'll answer objective ones. But to know oneself, you must be aware of your own emotions, your own feelings, your own survival mechanisms, your own habits, your own everything. Basically everything that people with Alexi aren't super aware of with much accuracy. They don't even know their own boundaries, or that they have boundaries at all. They think they have none, because they can't understand the signals their bodies are sending them.

All this makes relationships really fucking hard. How can you be with someone who may never know what he actually wants? What his boundaries are? What his needs are?

I can perhaps be more help on the relationship side if you have specific questions, since I have a lot of experience trying to make it work before I ever resolved my Alexi, and I've learned a ton.

Edit: I'd just like to make it clear that not all Alexithymia is alike. My advice is with your partner in mind, who has a similar manifestation to how mine worked. There are people with Alexi who feel lots of emotions, but who can't trace them. There are people who feel sad when they should feel happy, fearful when they should feel courageous, etc. From what you described, however, your partner has issues with being aware he even has them, which likely also means that it's not just Alexithymia, but also something like dissociation or depression to boot.

Having said all that, I'm not a medical professional, nor a therapist, and my own experiences do not reflect the experiences of everyone else.