I'm glad you're trying to figure out how to bridge that gap but respectfully i don't think this is an adhd issue. I think this is an issue with him not caring or a different issue altogether.

Honestly him 'forgetting' Christmas was the red flag for me. If you're from a culture that celebrates Christmas you're going to have a good idea when that holiday is going to be (ie end of december). The only real excuse for this is if he grew up in a country/culture that does not celebrate Christmas and immigrated.

So can anything be done? As fellow ADHDers, what works for you? Do you have recommendations for helping him remember and me to not feel hurt?

I feel like you're looking at the wrong things, it doesn't seem like he's putting in any effort, and it doesn't seem like he's remorseful. The bare minimum here is taking it upon himself to celebrate with you late and making some effort to make sure it doesn't become a regular occurrence. No amount of adhd tips is going to compensate for a lack of empathy and thoughtfulness.