I agree with Lisa...always trust your gut, even when the vet can't find anything. You know your dog best. It is young for a disk issue, but not impossible. A lot of it depends on genetics. Don't know if you got your little one from a breeder or rescued him, but if the former, you might check back with them and see if they've seen this in any other of their pups. It could even be something like a parasite, or maybe anemia? (I'm assuming they've done full bloods?) It kind of sounds like a GI issue to me...sometimes there are hidden ulcers or other abnormalities in the gut that xrays don't pick up. Years ago a friend of mine had a Springer Spaniel with similar symptoms. They finally ended up doing exploratory surgery and discovered a hole in his stomach that was hidden. Stitched it up and he was fine. He might have ingested something like a small wire or pin that's also hidden on the xray. Never hurts to do allergy testing for sure. At the very least, if you access to a vet who does acupuncture, it also might help. I know how frustrating it is to not be able to come up with a reason for what's going on (ask anyone on here about why my Alex was called "mystery doggie" ), but two IS awfully young for this. It never hurts to get a second opinion from another vet either. Worth a shot...sometimes they see something your regular vet missed. Sending a ton of healing . Please keep us posted, ok?