I had an 8lb SWCAH boy who did not gain well. He was maybe 18.5lbs at a year, at his year check-up the nurse wrote on his sheet 0% for weight and then erased so I would not feel bad.  I was worried at the time,  he is now 5 years old, still on the skinny side but very normal.  Not a real big eater still,  he gets full really easy.  He is active and happy etc...

I say if the levels are in good control try not to worry so much about the precentiles for height and weight.  I know it is easier said than done,  he is still quite small for his age, but it does not slow him down at all.  I have an older SW CAH boy, who is also slim now,but was chunkier as a baby.

Good luck to you.