Thank you. Well, to start off with, if he's being teased or bullied then you should speak to the teacher or principal of the school to see if they can be more aware of what is going on. However, the bot***** *****ne is that if he will always be short, then this is something he will simply have to develop a thick skin about. We live in a society that rewards tallness and thinness, that can be very cruel to short people or heavy people.
What you will want to do is help him develop the things he is good at, and that will help build up his self esteem. When someone is good at something it builds up their self esteem. Don't let him shy away from sports, football teams, hockey teams, soccer teams all make use of shorter players.
Talk to him about being short, ask him how he feels about it. Ask him why he thinks people place so much emphasis on being tall. As him to tell you the pros and cons of being smaller/taller. Take this as an opportunity to show him how racism, size-ism, bigotry, etc, can hurt people.
Bullying is a real problem in todays school and many times the school doesn't know how to handle it, so they downplay it or try to blame it on the ones who are being bullied. Many times kids will bully to enhance their own position. He will need to learn how to act strong and not let comments about his size bother him, because if he's going to be short all his life there will always be people who will have something to say. Most of the time, even the "nice" kids - will either join in or do nothing. For one thing, they have their own position to worry about, and if they defend your child, they may become the new target.
As he gets older, the bullying can turn into real physical abuse, vandalized lockers, obscene graffiti, vicious rumors, having their belongings stolen or destroyed, internet slander campaigns, sexual harassment, near-total ostracization and these are things that happen all of the time. Things that would be considered "violent crimes" if the public schools weren't held to a different standard. Bullying can make a child not want to go to school, to be sick all the time, to worry constantly, to make bad friendship choices, get bad grades, etc.
Talk to him, REALLY LISTEN to what he's trying to tell you. Do WHATEVER you have to do to protect your child, do not depend on the school to do it. If your child sees that you take his concerns seriously, he will be more apt to talk to you when there is trouble. A good book for him to read might be Judy Blume's "Blubber". Its a 4th grade book but he could try and read it, it captures that wolf-pack mentality that grownups typically seem to "just not get".
Look online for other books about bullying and teasing, about being short, about self esteem. It's not someone's height that makes them the person they are. Encourage him to do well for himself. I welcome your thoughts, let me know if you want to talk more