Does he seem to limp more on particularly rough surfaces? Like gravel or concrete?
Take a flashlight and very closely scrutinize each of his toe pads. Look for small flat shiny spots, or raised hard areas, or a tiny pinhole... anything that is not normal paw-pad texture. It may be a corn developing. If you see anything like what I described, take the affected pad between your fingers and give it a gentle-but-firm squeeze from the sides of the pad. This may elicit a flinch or a yelp (may want to muzzle before this exam). If so, it's very likely a corn.
They can be hard to spot, especially if you've never seen one before.
If it is, there are a few options for treatment which can be found here in H&M by doing a keyword search for "corns."
ETA: It's not unheard of for vets to misdiagnose corns, so don't let the fact your vet hasn't brought this up make you dismiss the possibility out of hand. Might save you some xray expense...