What a cute boy! Glad you decided to take him in, he looks like he's going to be an excellent cat. Love the name, too <3.
I will say, though, that I think the reason that people were getting so defensive about the whole "purebred/not" thing is because some people would decide that their cat is some rare breed and not have them fixed or treat them differently because of it :/. No offense, but if you picked up an unpapered, unaltered, purebred dog at the shelter, breeding them would be irresponsible because you don't know their history, etc.
But you're obviously getting him fixed either way, and would treat him the same no matter what he is, so you can call him whatever you want. I mean, imo he's just your typical cat - mixed heritage, mixed background, all awesome. Definitely not a Khao Manee - they have a very different face shape than your boy, but he's still beautiful!
Unfortunately, there are no genetic tests for cat breeds :/. Unlike dogs, differences in cat breed are very subtle for whatever reason. But he's a wonderful boy, and I'm sure you'll love having him <3