@monnie24, I second the notion. Sanjay is our paed for DS (8wks) he was worried about his weight initially with not putting enough on according to the charts then I reminded him of my older DS who is 9 and is so skinny it's not funny but he eats more then me and struggles to put and keep weight on. If you look at family genetics that to can play a massive part is how our kids turns out. My DP is 6ft7 I'm 5ft11 new DS is 4kg @ 8wks and 58.5cm already he has grown 7.5cm and is only 900gms above his birth weight. But he is happy healthy sleeps passes all his exams and Sanjay is happy with him. I give him a top up of formula some days between feeds when he needs it.
See what your paed says to you on wed. If you do have Sanjay he will write everything you discuss down for you and give you a copy and you can even txt him if your majorly concerned and he'll get back to you