supporting my boyfriend and his autism

hi I'm Hannah and ive been with my boyfriend for 10 months now and I think I'm doing kind of okay with supporting him and his autism. I just think that I need to work on a few things like helping him feel okay communicating his feeling with me. If we have a disagreement or something goes awry or even sometimes after just an innocent off hand comment he will completely shut down for like 20mins minimum just withdraw completely, lay down in a ball, silent, eyes closed. It passes with time but often I am too worried of triggering something again to try an discuss what happened and even if I do he often tries to avoid it completely. even recurring things like why he refuses to come to my house anymore when he used to be okay with it are completely off limits and I don't know how to approach this issue. Does anyone have any ideas on how I can help him open up to me?