Depending on where he is stocking up, it could be a joint thing. X rays aren't going to tell you anything. I suggest getting a scan done that detects inflammation in the joints, I don't know how many vets are trained to read them yet, but it's a really cool technology.
Otherwise, I would just cold hose the area and bute. Get the vet to flex him first to rule out joint issues. If he trots lame, or is uncomfortable in the flexions then get a scan done where he shows discomfort. If it ends up being joint swelling then there are a lot of differnet options, but I really caution against steroidal injections, although they provide short term relief, they actually degenerate the joint.
If he flexes fine then continue working him, and cold hose him after riding. Bute is also good because it is an anti-inflammatory. He may have just slipped, or it could be his legs reacting to harder ground.