How old is he? Depending on his age, he really might think it's catching. My sister teaches first grade and you'd be surprised at the things they think they can catch. They also say things without intending to be mean--it's just how they are.
Good job not engaging with the staring. He really may not understand that it's not okay. Direct explanation and expected behavior is always good, should you wish to interact with him. Ignoring works, too
I was born with one hand. I have about 3 inches below my elbow on my left arm. I have dealt with my fair share of teasing and stares as you can imagine. All you can do is rationally explain to that child what happened to Polly, point out that she can do everything every other dog can do, and leave it at that. Many times this works immediately. If it continues than I imagine he is not being taught tolerance and acceptance at home and there isn't much more you can do. Some people just can't accept differences in people or animals. Me and Polly can be twinsies
Is he teasing or just being weird about it, like he has zero home training and no one ever told him to shut up and not voice his every stupid thought? Unless he's being menacing in some way, I don't see how this makes a difference to your dog. It bothers you. Treat it the same way you would if an eight year old were teasing you. You're an adult being harassed by an eight year old.
The best thing you can do is act bored and rise above it. You are probably not going to change the world view of an eight year old. Yeah, yeah. Three legs. Just like yesterday. Jeez, kid, you need new material. Eye roll. Bored yawn. Keep it moving.