While I can understand the sentiment, what is a proper way to indicate to a average human being that it is expressing behaviour not associated with normally developed adult person?
It looks like he is behaving like a normally developed adult person. Unless you consider every partner in a divorce to not be normally developed, nor anyone who's ever expressed hostility towards somebody else for what they appear to think of them. Most adults do these things. That makes it normal.
Use your words, kind of like you just did. Throwing a disorder around as an insult is always the wrong choice.
"Immature child" would have been fine. You may not be aware that autism is a spectrum disorder, with a very broad spectrum. I've got my troubles, but I've got a degree and a career and a life, and I do get tired of people assuming that autism means I need help getting dressed.