
My Rottie boy is skinny and much smaller than my girls. We rescued him (well took him off one of our employees when we saw the state of him) as a 5 month old. He was poorly bred, had never been wormed, or fed properly, and looked like an RSPCA advert, due to his ribs, spine and hips sticking out. He'd been attacked by a bitch who was just protecting her pups, and looked like an English bull terrier, as his nose was so swollen and septic.
He's a nervy boy and is fear agressive, and still ridiculously skinny. He's very healthy now, but it took 12 months to settle his stomach. We just can't get weight onto him, we think he burns it off through nervous energy. He's our boy though and we love him, even though he does have his moments!
If your boy is still young he may just be going through that stage where he's growing into his body, not sure that makes sense, but when I used to own Bullmastiffs, they went through growth spurts where they looked to be all limbs and skin, then they'd fill out to fit it.