I have two SW CAH boys,  my second was really small his first couple of years,  weighed 8lbs at birth and maybe weighed 17 or 18lbs at his first b-day.  He just turned seven and he is in the 10% for height and weight, he was not on the charts at times.  I do not worry about him anymore,  he is fast, funny and full of energy.  He did not eat much when he was little now he eats everything. I even have to limit his food at times.  It drove me crazy when he was younger.  He has been under good control most of the time,  had a very hard time with his sodium at first,  he took 4 florinef for the first year.

His brother is slim but super small,  especially as a baby but he was overtreated by a bad endo so  who knows what he would have weighed.

Try not to worry.