Oh, poor baby! Dogs can only tell us when something is wrong by their behavior. The first thing you need to do is get him checked out with a vet who can check him for orthopedic/neurological/GI problems. He's not jumping due to pain or something preventing his wanting or ability to jump. He could have injured himself somehow from falling off the bed, couch, steps to missing a jump or suddenly developed a little arthritis in a major joint or tumor on his spinal cord to merely having bad gas, tummy troubles.
Dogs usually only react to sudden or surprising pain but go stoic and instinctively try to hide their pain from their fellow pack member(s) for fear of being abandoned or attacked in their weakness, a holdover from their natural wild side. Despite centuries of domestication, it is surprising how much of that stoicism remains in dogs who almost always fail to show how much pain they are experiencing, even to those who love them the most.
That is why a vet visit is always in order anytime a dog's behavior changes, from suddenly growling, snapping at us to failing to do something he always does to isolating himself, shaking, sudden clinginess, refusing food/treats, toys, etc. Just because a dog can be incited to eat, run or play at times doesn't mean he is well. Call you vet and get him examined, x-rayed now. He could be in pain and waiting for you to help him.
Please keep us updated on what the vet says and how your little guy is doing. I'll be praying for the best outcome.
__________________  Jeanie and Tibbe One must do the best one can. You may get some marks for a very imperfect answer: you will certainly get none for leaving the question alone. C. S. Lewis |