One of my dogs, Diesel has a thick well built torso big muscley neck, and for his size a wide chest, and well defined arms but has a small head and twiggy hind legs and buttox he's 8 months old. Will they fill in or will they stay that way?
My girl was kind of the opposite. Her hind end developed really quickly and it took a lot of running to get her front stronger and we're still working on her core. Have you tried any targeting exercises to try and build up his hind end?
Unless you purposely acquired that pup because you had knowledge of the breeding stock and their ability to produce what you are looking for, you're pretty much rolling the dice. Some dogs grow up disproportionate. Kinda get what you get sometimes