I went through something similar, but my daughter was born under 5 lbs due to an umbilical cord problem so she's small to begin with. But her weight dropped down on the charts pretty drastically so my dr sent her for blood testing at the U of I with an endocronoligst.
If you go, they will want to know specifically what he eats, how much and many times he eats per day and what he likes and doesn't like. THey will probably ask you if celiacs disease runs in your family and even if it doesn't, it will more then likely be one of the things they test for.
All of her stuff came back normal and they encouraged to push higher fat items. Avocados are higher in fat, I give her hard boiled eggs or deviled eggs, I make shakes at home with flax oil in them (you can't even taste it), she eats a lot of pb sandwichs. Pediasure is another option but at one time I spoke with a nutritionist about my son and his picky eating habits. She recommend using carnation instant breakfast because it was similar to pediasure but less expensive. THe 3 original flavors are best, because the mocha and other flavors have sugar listed as the first ingrediant. They also recommend putting butter on her veggies and such, but I didn't agree with that. Some butter is ok, but on everything??
Unless he has some problem that shows up in his bloodwork, they probably won't recommend hormone therapy. Good luck! He's probably just a small boy and might catch up later. But at least if you go to the appointment, you'll know if anything's wrong rather then being left to wonder.