I have an almost 6 year old skinny CAH’er  he was not on the chart for a bit for weight...he is very active and happy and eats great now.  Still smallish but he can and does everything.  My older  SW CAH’er is more average size,  all kids CAH or not come in all different shapes and sizes.  I would offer healthy food choices and not worry about the weight thing,  there are a lot worse things than being skinny.  As long as they have plenty of energy and their levels are good, try not to worry.  I know it is hard though.  The nurse would not write down his weight % at a check-up because it was seriously 0%.  He is probably 10-25% now and eats a great variety of food.  I did give  him carnation instant breakfast in his morning milk,  I think he eats better now that he does not fill up on the fortified milk.

Good luck and Hang in there!