+1 and also chat with his therapists for more suggestions.

I think the most respectful thing for your son is to be honest. Not in the adult way of saying "Those are 'stims' which is short for . . . and is a symptom of . . . ." but in a way that honors the underlying reasons why he uses them, and also acknowledges that the stims are probably filling a familiar function. Of course, I don't know the exact reason why your son stims but it might be something like this:

"That's something he does when he's excited (or nervous, or whatever), it helps him stay calm. Coming to the park is pretty exciting isn't it?"

or

"He enjoys watching his fingers, what are some things you enjoy?"

or

"I think it feels good to him, it's something he likes to do. He likes to do other things too, like run. Do you like to run?"

I would explain that he does that when he is feeling nervous or shy. It makes him feel better. I might also say that he feels nervous or shy when he meets new people.

I wouldn't try to explain it. If a kid asks directly, you might say, "That's just the way he plays."

DS is 3yo and diagnosed with high functioning autism. Recently I have noticed that his peers are already starting to notice some of his differences. For instance, we have been at the playground and another child will want to go play with him. Aside from the fact that he does not really respond appropriately and shies away, he tends to start stiming because I think that is a little stressful for him. I have heard the children ask their mothers "what is he doing?" One time we were able to just breeze over it and encourage playing together (with lots of help, thankfully a very persistent playmate.) Other times I wish I had a good child friendly explanation. Once I explained his mouth stim as him exercising his mouth. It did not feel quite right though. Any thoughts? Thanks!