My son is an only, as am I, and he is so sensitive to being teased. Any advice?

Where does this teasing happen? Yes, we all need to develop some thick skin to get through life and your son's reactions are too strong but - where are the teachers in this? Teachers should not tolerate teasing. Teasing is a kind of bullying - that your son is overly sensitive likely earns him extra attention from these emotional bullies and you should work with him on that but - where are the teachers? The bullies need to be worked with too.
His classroom teacher for even the rest of this year as well as next year should issue a 'no teasing 'round here' rule. Teasing doesn't make for a good school day for anyone - the teased child or the bully child who's learning that it's ok to tease another child. Indeed, that your son overreacts is what should make the other children STOP teasing him.
Their teacher needs to explain to them all that we all react differently and some of us react strongly. Just as we are all learning how best to be - and it's best to respond thoughtfully when someone says something mean - it's even better for us to be a school community where no one says mean things. "Whatever do you mean, Billy, by saying these things to Jon? Do I speak that way to you? What's your point? To torment Jon? Why?"

We need to never bully bullies -that doesn't help - but we can and should make them squirm a bit. I don't disagree - it would certainly help your son to learn these hurtful words are only words that only hurt as much as he lets them. But your son and every other sensitive child will only learn those lessons when they feel safe and supported by their teacher(s) as well as by their parents. These teasing children should not be given the opportunity to ride roughshod over your sensitive son. His response to it is not helpful but the first thing that's gone wrong here are the other children and the teachers that allow their teasing.