A break is one possibility. It may also be the result of a slipped tendon or another issue. You should have someone hold him while you examine both of his hock joints simultaneously. Look for differences in the normal vs. injured leg.
My avian vet once told me "put two chicken bones in the same room and they'll heal together." Look out for broken bones in chickens is usually pretty good. Even if it's not broken, with leg injuries treatment tends to always start with isolating the bird and restricting movement. Use a crate that had enough space for him to turn around, and for feed and water. You can give him ibuprofen for pain and inflammation. Dosage is one 200mg tablet, crushed, per 2 cups of drinking water. Some people will tell you that aspirin is safer but I have personally discussed this with my avian vet and she says ibuprofen is the safer option.
