Is wearing glasses not as smooth as you though it would be for your kid? Are the mean kids out to get him? No need to panic. Chances are your little guy won't be scarred for life because of it. In fact, teasing can be great for character building, if you can help your child get past the initial hurt. Even better, once your child learns to ignore the teasing, the bully will leave him alone and move on to something else.
Explain that wearing glasses is perfectly okay and that the other kids are just being mean. Keep it short, make it funny and don't make a big deal of it. Say something like "You're cute with glasses," or explain that he will no longer get headaches by saying "No more head ouchies now, right?" Or bring out his favorite picture book and show him the glasses can now help him see details he couldn't see before with things like "Look at that small ant on the page!" or "Can you see the tiny letters now?" Don't ignore his feelings; if the teasing hurts, it hurts. But only indulge it for a few seconds and then focus on something else. If you make a big deal of it, so will he. Teach your tyke to do the same when the teasing occurs: Just ignore it. The more he responds to it, the more the bully will tease.
Teach your little one to respond with humor if ignoring the teasing doesn't work. Tell him to smile and say something like "Yeah, that's me, four eyes!" By acknowledging the tease, he is taking the power away from the bully. Now the bully will have to come up with a new tease, because the usual one no longer works. He can also respond with something even more basic like "Leave me alone." Tell him to walk away after saying something -- no fighting, no crying and no screaming.
Get yourself a pair of glasses and join the club. It doesn't matter if you need them or not. You can buy a cheap pair at any pharmacy or discount store. Just pick the one with the lowest possible lens power and make it fun by picking the goofiest frames you can find. Some stores sell eyeglasses that are basically just plain glass; these are usually made to protect your eyes from debris when riding a bike, for example. A toddler might not understand "It's okay to wear glasses," but showing him that Mom wears them too -- and she's okay with them -- says more than words can.
Find examples of famous kids in cartoons or books that wear glasses. It doesn't get any cooler -- or smarter -- than Harry Potter. Or go for the fun, quirky characters, such as Waldo, Stewart Little or Chicken Little. PBS' cartoon Arthur is a great example because he's, well, just an ordinary, everyday kid. Even fluffy Sponge Bob wears them sometimes. Pick the one character or cartoon your kid loves and then print out pictures from the Internet where the character is wearing them. Seeing one of his "heroes" wearing glasses might help him deal with the idea better.