@cbgirl123 My 3 children were very active in sports all through elementary and high school- they were swimmers and had long daily practices. They were all lean and picky eaters.
In high school my oldest daughter started having problem not wanting to eat after practice- she didn’t feel well, no appetite etc. She wouldn’t listen to any advice either- stubborn. As a result she did not perform as well in the pool and couldn’t understand why- refusing to listen.
Giving protein drinks is a good idea/ there are quite a few supplements nowadays.
I agree with Teresa that you should ask his doctor about his nutrition state and growth. At age 13 there is a great variability in growth among boys. Some may be far into puberty, while others are just beginning. That’s why one sees a lot of boys looking like young men and others still like boys. Everyone has a different growth pattern. The boys who look more mature may not grow much more- however the younger looking boys, like your son, will grow several inches still and probably will end up taller than most.
As long as you know that he is declared healthy by your doctor, he has years of growing still.