yeah, i'm 5 foot and gained like crazy when i was little, and my cousin was in the 5th percentile when she was a baby and is now almost 6 ft.
so, i dont think the charts have a lot to do with how you end up. babies just grow at different rates, if he is gaining weight then i'm sure he is fine. as long as he isnt losing, you know?
i wish my baby was tiny, she especially looks huge when i am carrying her because she is now HALF of my height!