S.- This is genetics and as long as he is growing, there isn't much more you can do.
Make sure you are providing overall good nutrition and he'll be ok. Kids can be mean to anyone that is "different" or what they perceive as "different" regardless of what that is.
That being said, I know exactly where you are and what your fears are. My son fell off the growth charts at one point - theoretically there were no kids his age smaller than him. He was a normal size at birth, so that wasn't an issue. However, we have family that ranges from 5' to 6'8". So there's a wide range that he could have fallen into.
When he was about 11 YO, our pediatrician noticed that there had been no growth over 2 years. Because of our HMO's we'd had to change pediatricians nearly every year. We started to see an endocrinologist to see if we could use HGH with him. HGH is only effective where the body isn't making it naturally. If there is any natural HGH, the child could actually have his growth stunted. Over the 2 years we were working with him while they made sure he wasn't growing - he grew all of a half and inch and he was no longer a HGH candidate.
My son did well during this period and became extremely independent. A lot of that came from my DH and myself telling him that he could do whatever he wanted. We made sure he gained confidence thru a lot of sports (karate, soccer and hockey) and he did. He learned that even with his small size, he could compete with them. We knew he'd be ok, when he came home one day in middle school telling us about a bully who he "hipchecked into a locker & he won't be bothering me again."
Then, as a freshman, we had noticed he was doing some growing but not how much. We had to take him to the pediatrician because of pain in his legs. His dr said that he had the pain because in the last year he had grown ELEVEN INCHES and gained 20 lbs! We were amazed. No wonder I was buying clothes every month for him! He was still smaller (about 95 lbs) but gaining on his friends.
Now, he's 5'8" and still only about 115 lbs at 22 YO. Actually, he's the tallest in our house.
It's hard to wait to see the outcome of what our children will be, Sometimes, we need to.