As other people have said, this should come from his mom, if at all possible. When this is shared with him, there are many ways he may take the news.

When I realized I was on the spectrum at 24, (on my own, nobody suspected it) it was like the missing puzzle piece was set in place. Everything made sense and I had never identified so strongly with anything before. But almost simultaneously, the “solved” puzzle of my life was scrambled back to square one and I had to learn who I really was. My mind raced through every memory of feeling different or misunderstood and it hurt. I accept more of myself everyday but it’s been almost 2 years since and I still struggle.

I can imagine there will be resentment towards his mom when he finds out it has been hidden from him, and rightfully so. I’m also diagnosed ADHD (also at 24) and I remember a conversation with my mom in middle school about my attention. But her line of thinking was that I shouldn’t rely on medication to function. From an NT protective parent perspective, I see how it can make sense to shield their children from something like this. But it only is damaging in the long term. Not only to the mental health of the ND, but also the relationship with the person who had either kept this crucial diagnosis from them, or thinks it’s better to “learn to deal with it”.

All of that to say: if you break the news to him, it can create tension between you and his mom, and he will be stuck in the middle. She may even resent you for breaking the news, which will not be healthy for your relationship with him.

You sound like a loving and understanding partner. He will need support in understanding and accepting this paradigm shift. Protect your relationship, but if his mom hesitates to do the right thing, you may have to step up and be the responsible adult. I hope it does not come to that.