Once you rule out (or rule in) hip dysplasia, I'd be talking to a canine physical therapist in order to start building muscle in the rear end. There are plenty of exercises you could be doing (even training them as tricks) to help strengthen his rear end. But don't start this without knowing (medically) the state of his bones & joints.
Pimg - DOB: 2/06, CL3, CL2, CL1, UJJ (x2), HIT, CA, CGC
High Jinks vom Neuanfang - DOB 9/12 (Gotchya Day: 1/23/2013) agility superstar in training