A word to the wise:

[conuly]

Your autistic child does a thing, he does not have a behavior. If you want to ask why your child has recently started stimming in a new way, the words you want are "Why has he started doing this?" not "Why does he suddenly have this?"

The latter phrasing is awkward and, more importantly, is not something I can imagine you using for any other person, so it's a little shocking that the person you're using this weird circumlocution with is your own son.

(Also, the person best poised to answer that question is your son, not the internet.)

This has been an open letter to an unnamed person. Sincerely, me.