My dd is missing her hand.
Firstly: They will get comments. At reception year they tend to be interested rather than nasty. Encourage your ds to think of them as interested questions, rather than people being nasty.
Secondly: One of the most positive things you can do is encourage your dc to answer honestly without getting upset. "I was born like that" was dd's answer. If she liked the person she would go further into talking about what she could do etc. Actually you'll find that "I had an accident" will be easier for them to accept than "born like that".
Thirdly: How he reacts now will spell out how they will react to it in the future. If he reacts as in "no big deal, so what". Then they'll probably forget about it.
Lastly: He will get nasty comments at times. He will potentially get people staring as they go past. He will have questions about it all the way up, even in adulthood, if it is still visible. In some ways learning to deal with the questions when young, and they're very matter of fact, is better than trying to protect them at that point.
I would have a word with the teacher. Then if you get a child who keeps pestering (you occasionally get a child who really finds it fascinating) they know to tell them to back off.
Some people go for the approach of parent going in and talking about it, and saying how they are just the same as everyone else is. Personally I think that has the potential to back fire-after all if they're the same as everyone else, why do you need to tell them? I certainly think if you're likely to get upset, that isn't a good idea.
Some people like the book "Wonder" for the child. My dd didn't. Her comment was that it gave the message that if people still didn't accept you when you look different then that's because there's something wrong with you inside too. However you may find it helpful when he's a little bit older.
Another thing potentially to do is look out for people like him. Harry Potter is the obvious fictional example, but maybe there might be a soldier on the news, or someone on a quiz programme. "Oh look, they have a scar like yours, isn't that fantastic!"
Make it something special to be proud of, rather than something to be hidden away. That way he will have the inner peace to be able to cope with any negative comments.
Dd was always so proud to see someone else like her.
You can also get dolls made "like you" if that would help.