Well done, to both boys!
I was bullied viciously in elementary school - punched, chased, spit on, kids swearing into our answering machine, vandalizing my bike... it was brutal.
My daughter was born with a large haemangioma on her upper lip. It pushed her nose up and dragged her mouth down. I remember saying, "It's okay, because all her life, if someone teases her, she'll believe it's because of her birthmark." I had been an ordinary-looking little girl, and when they teased me the part that cut the deepest was knowing that they hated me not because I was different and not because I wore glasses or talked too much or was too clever, but because I was just being me. I desperately wanted to believe it wasn't really true, but I knew it was - I was a dork, a geek, a loser, just like they said.
A birthmark, though? That can be dealt with. Teasing someone over something they were born with just reflects badly on you, not them.
My other thought on the issue was glasses or no glasses, birthmark or no birthmark, kids will find a weakness and exploit it. So, build up your child's defences. Be kind, be reasonable, don't tease! Let them make decisions for themselves, and respect their opinions. Don't bully them. Don't browbeat them. Don't humiliate them. When they come across these things in school, they should know deep in their core that it isn't right. And how will they know that? Because these things - bullying, teasing, power plays - won't exist in their homes.
We bully-proof our children by never bullying them ourselves.
It also really helps if you have a good, sensible school system with excellent leadership.

In the end, we lucked out. Great teachers, strong anti-bullying initiatives, student involvement... actually this story basically says it all:
Ninth grade, my daughter began bickering with another ninth grader in the locker room. They were the youngest girls there. The others were older - juniors and seniors, and they were ignoring the two girls. There were no teachers in the room. It got heated, and the girl said to my daughter, "Yeah, well... you're ugly!" Immediately, the older girls turned around, as a group, saying, "You can't SAY that!" My daughter came home completely amazed that girls she didn't even know would step up to defend her like that.
I'm very happy to say that both my kids had a much better public school experience, than I did.