I agree with others that a cheerful he's our son would probably be best with that specific question. "What do you mean?" and similar can also work if you've the energy and feel safe doing it, but there are situations where just being breezy, confident, and moving on will be best. I think it's more important that you show confidence in the issue than always facing down people.
I'm mixed race, and my siblings and I all look very different to each other and to our parents. There were a lot of milkman/mail man/etc comments growing up. My mother would laugh along with it (along with similar comments made about my father who is very different in colouring to his siblings) which was a bit of a headfuck because it fed whispers around us. I was in my thirties, seeing a passport photo where I'm the spitting image of him in facial shape, before I could clear that doubt. I wish my mother would have just respected us enough to be clear on the topic with others, even if her tone had been the same.
Thankfully, my spouse and I - who are very different in appearance and ethnicities - never had that comment, though I have had a few who make remarks about how 'great' it is that they have my colouring (they don't actually - they somehow look very similar to each other, but have an entirely different skin tone and hair colour to me) rather than their father's ginger/pale colouring. I have spoken up that I find that rude, but I doubt it made any difference as it seems people think they're being nice to me by saying that - how insulting my spouse and my taste in partners is nice, I don't know, but it was a thing when they were younger that I'm glad has pretty much stopped now.
We do get a "what are you?" or remarks about us 'obviously' not being from around here (my kids have lived in the same city their whole lives), at least near our old place. We've moved to a different part of the city and haven't had it yet. With that, I've worked with them on how they want to handle it as they dealt with stuff and grown older. They all have different views on it, so it's mostly helping them feel confident and stepping in when some people won't let it go which sadly does happen though it is rare. I had it a couple months back, just before we moved, ended up having a man shout at me in the streets because I'd asked him to please leave my son alone after my son had answered his questions and the man refused to believe him. I got called all sorts that day, but thankfully as I said, that's pretty rare.