If he's really active and really lean, he probably just needs to eat a LOT. Make sure you do the proper bloodwork and worm checks (I think you already checked for worms), but if the vet says he's healthy, I'd stick with raw and throw him more animal parts! Mine seems to have a very high metabolism despite not being active enough to justify the amount he eats for his size, but it's what works for him. I increased his portions and about every other day or so he gets an egg. I make sure each meal has plenty of fat. If the day's rations are extremely lean, I picked up some pork skin with fat on it and cut up very small bits (about 1" x 2" and a centimeter thick, give or take), just enough to add a shot of fat to a fatless meal. I feed him in his crate and leave him plenty of time to lick, crunch, swallow, and clean up his bowl and crate. It's mostly covered with a blanket so he feels really safe and secure. Make sure you're feeding your dog in a place where he'll feel secure enough to spend time eating.
For what it's worth, even in people, we can eat our brains out and still count all our ribs. I remember plenty boys back from adolescence and early teen years who've always looked skin and bones, especially at the local pools, but they eat enough to put sumo wrestlers to shame. I eat insane amounts for my tiny size and you can count every single one of my ribs, but I have weight elsewhere in my body. Does your dog have lots of muscle? He could just be a really awkward and gangly teen. I'd keep up the vet checks, keep a close eye on his overall health and behavior, and just offer lots of goodies and things to eat. Try variety- if chicken doesn't do it for him, try beef, lamb, turkey, etc. Renji usually prefers his meats with some sort of canned fish or canned veggie on top or some olive oil for flavor/coat. If the "sauce" isn't there, he'll take a little longer to tuck in. If your dog is doing great on raw (shiny and healthy coat, low or no odor, small poops, great vigor), I really suggest sticking to that! I do agree with posting a photo. Maybe he's really not so bad.
Check this dog out:
http://leerburg.com/cj2.htm See how beefy and solid he looks? Well scroll all the way down and see the second photo from the bottom. I remember when this was posted, he was in the teenage stage at the time, and you can see every last detail on him. Apparently it was just another "gangly growing boy" stage because he definitely filled out!