If you are going to continue to provide a family for your kiddo then you should look into professional help immediately. If kiddo is not talking to you, maybe he will talk to someone else who is has an understanding and has some experience with autistic children.
Its really up to you if you have the backbone for kiddos way of life. Just remember its the autism not the child who is behaving in such ways. Some days are going to be great and some will be hard. You sound like you want to continue to care for your foster child and you are reaching out to better help him. God bless you. you are beautiful and I pray you find the right answers.

Omg.. you are doing great as we speak.. you really got this. Don't let fear get in the way or cloud your judgement. I'm impressed!
Kiddo is very lucky you came into his life.

I know a lot about our personal autism journey and very little about adoption, except from some cousins. I am wondering if you think any of these behaviors may be reactionary to this placement truly settling in on him. He's feeling safer so he's really acting out. Not suggesting he might or might not be on the spectrum. My children with ASD did not have regressions although I have read that other children did. Mine were just always delayed and eventually diagnosed.

Sounds like you got a rude awakening to the world of autism. Not surprising as the general public doesn't see many troubled ASD kids in the media. I still get the "so your kid is a genius" comment once in a while. They never met her so that's supposed to be a compliment I guess.

There should be a trigger(s) to this negative behavior but you can't see it. Is he bored? Seeking attention? Acting out so he can bet moved again or thinks he will be anyways? Do you know much about his background?

Best thing us to change his diet. Take out gluten and sugar for 1 week and see if you notice a difference.