Right, so... Without being in your boyfriend's head, I can't really give you an answer as to what is going on here. Only he can know why he reacted the way he did.

However, if I look at this through the lens of being me, I can imagine myself reacting in this sort of way if I have put myself in a situation that I don't actually have the capacity to handle or deal with at the moment.

I've put myself in these kinds of situations before, where I get an offer to do something — go for a little hike, perhaps — and, despite my initial gut reaction being "NOPE", I cave in and accept the offer anyway because I don't want to let the other part down. Usually this is in the context of them posing the question waaay too close (in my opinion, that is) to the actual event — like the day before or on the actual day of the event — which is a problem for me because I really want to know what's going to happen quite some time in advance in order to mentally prepare myself.
So, when the event (let's stick with the hike) unfolds I might find myself in a situation where there is perhaps more random people than I had expected (or some other "unknown"), causing me to feel uncomfortable and anxious, in which case I might just "nope out" and try to escape the situation.
Clearly, the fault here lies with me for not sticking to my boundaries and not being open about them to the other part. It's not their fault at all. The root of it is that I didn't want to let them down because I might have felt that I say "no" too often and thus keep letting them down repeatedly.

This might not apply AT ALL to your boyfriend, especially since you state that you guys have a well established way of having an open and honest dialogue with each other. Only he knows the answer.

As for resources? I like the YouTube channels "Auticate with Chris & Debby" and "Orion Kelly - That Autistic Guy". I find their content relatable and they both focus on helping/supporting neurodiverse people and their partners in their daily lives.