I agree w/ JeanneHall- someone is probably bullying him causing him to feel that way. If it continues I would have him seen by a Child Psychologist. They are good at having kids open up about their feelings.

Also in a silly context most the time it would start with baby talk from one of them, saying peek a boo or pretending to cry like a baby going wah wah (not actually crying out )

Depends on the age it might just be wanting attention and pretend play for a reaction, if you react big to it you get more reaction back and the cycle repeats. Both my kids asd and not have went through that phase, (at different ages) it can be tough, but it also didnt last long, we talked about babies to them both and about how old they are. Example : oh your being silly! Babies are so tiny did you know you are so Big now! And so forth and also praising them for being a big boy. Can you also identify why he is having a meltdown? How you react or right before? Has he seen other younger kids have meltdowns somewhere he goes frequently and the reaction they get from it? Like perhaps copying something? It depends on context and the situation if it is bullying can you talk to them about it? Theres many reasons that behavior could be happening or something they watched on a show as well. If its bullying kids typically refuse to go to school, or become upset having to go among many other reasons so youd notice quick if something at school was triggering this.

Is he in school? If so, he’s probably reacting to bullying by others there. If that’s the case, it should be reported ASAP to the teachers and administrators. Then, remove him from the school!

This is the number one reason I believe home/life schooling is the best for Autistics: they run a high risk of being bullied, teachers and administrators always look down on them, and nobody understands or appreciates them!

How old is he?