Toddler has almost no body fat at all. Doctor doesn't know why. Anyone have experience with this?

I posted this in r/beyondthebump, but I figured parents in this sub have a wider age range of kids and someone might have been in our situation.My 2 year old son (28 months) had a check up today. The doctor noticed that he has an extremely low amount of body fat. The issue that confused him is that my son isn't super underweight or anything. He just doesn't have body fat. He naturally has a bit of a more "muscular" (I use that word loosely) body type.He's always been a strong kid. He was able to escape his crib at 15 months and we had it on the highest setting. We had to get rid of baby gates because they became too much of a risk factor with him climbing them. He has two older brothers and likes to try and keep up with them. He does appear to be stronger than the average two year old (he holds his own when wrestling with his 6 year old brother) but I attributed that to him having brothers and being around a lot of older boys. They do something and he wants to do it too. He can hang from the monkey bars with ease and can throw pretty far. He was walking early and pretty much avoided the whole "toddling" stage. Nothing super out of the ordinary, but the pediatrician did make note of it. He's extremely active (go, go, go from the moment he wakes up until bed time) and eats a lot.We were informed that we would recommend a specialist some time next week. I'm pretty much freaking out over what this could be. I know body fat is important and I'm worried about there being a major issue here. Can anyone ease my mind until we get the next phone call? Any ideas on how to increase his body fat percentage? via /r/Parenting http://ift.tt/2rH2ER8