Without really knowing what you're describing.....I'd say your best bet is to get him to the vet. Describe what's going on, be as specific as possible...like does he seem to be throwing his back end to one side, is he limping, does he stumble, does he seem to be in pain, etc.....or even just call the vet first and explain what's going on. It could be as simple as he's just a lanky, clumsy puppy, and as complicated as possible hip displaysia (I don't think I spelled that correctly) or something else to do with his joints or with his balance. Even something like an ear infection could be throwing his balance off. Have you checked him all over? Like inside his ears, checking his sight and his hearing, felt his legs and back and checked his feet and stuff? Sorry I can't be more help, maybe post some more specifics and someone else can help....