Hi and welcome! Gosh, that's a hard one! With xrays ruling out so many orthopedic conditions, I'm hard pressed to come up with any idea of what it might be. Did Buddy start having these problems recently? Have the vets done basic blood work in addition to the xrays? I can't imagine how G.I. symptoms would make him hesitant to climb stairs and such, and as you say, he's eliminating well and I'm thinkig he's eating ok. I do know of people whose dogs have exhibited similar type symptoms that went undiagnosed. They went the route of doggie chiropractor for adjustments, therapeutic massage and utilized other holisitc treatments. Buddy's still a young pup! I hope you can find someone who can give you some answers!! Wish I could offer more help. Get well soon, Buddy!