At that age they are normally asking just to know and out of interest, not anything malicious.
My dd was born without a hand and about 4yo is the age when they would turn to her (sometimes after knowing her for years) and say "why don't you have a hand?"
Best thing to do is give them some basic answer that they can understand.
For dd, normally "I was born like this" returned the response "oh" and that was the end of it; never worried about it again.
Occasionally they'd comment "I was born with two hands" and again that would be the end.
Sometimes they'd ask further questions ("Do you have two feet?" was one, for example) The more matter of fact you can be about it, the more they just accept it as something normal.
I did have one child who really worried (me) about it, and I think they just couldn't get their head round it - I suspect if I'd said "a shark bit it off" they'd have found that easier to understand. Thankfully they came to me not dd, although that may well have been because she was more brusque than me!
So work out a standard answer that you or your ds (if he's able) to give.
"He finds it comforting? Do you do anything that you find comforting - do you have a soft toy at home?" might work. Perhaps just "it's something he does".
I'd avoid using the term "likes doing" because that implies he is choosing to do it.
But find something that they can understand in simple terms, and there's a very good chance they will just accept that it is something he does and not think too much about it.
I would be careful about making too much about it in school. Some parents (with dc like dd) like to go in and talk to the class emphasising that they can do everything etc. My observation over a number of years is that although this can sometimes seem tempting, it can highlight them as different and mean the children are looking for differences from the beginning - one dc I know whose parent did the above the children then pointed out (not maliciously but as an observation point) whenever she struggled to do things, or did things differently, for example.
However, I would speak to the teacher and let her know what was said, so she can be there to step in if your ds isn't coping, or questions get repetitive. Also tell them what you are planning on saying, so you can sing from the same hymnsheet.
And my ds had a friend who stimmed - at times quite noisily. The children normally just ignored it and accepted it happened.
On occasions (she also had Tourettes) she came out with something that they did find hilarious (the head in assembly once said "I'm going to shut up now" and she responded with "about bloody time") and even then they normally managed to mostly ignore because they knew that she didn't mean it. I was seriously impressed the way the 10yos managed to suppress their laughter on this occasion.