When penny had cold tail it happened that night after a long day at an event (and dock diving). she walked fine, her dial was just very sad and limp. what do you mean favouring his back legs? his front is sore? Sounds like much more than cld tail (or maybe extreme version of?)
hard to say how long i would wait to go to the vet, without knowing if that was "a lot"more exercise or how bad he's acting and if he's a wuss or stoic. Pen is a wuss so I can't run off to the vet (well I did when she tore he CCL but that was an extreme symptom) each time she limps. But other dogs rarely show pain so those ones I'd be quicker to go in if they showed ANY sign of discomfort. For a limp (which this is not so not entirely helpful) i do full rest for 7-10 days then resume activity. if limp persists I go in (or depending on severity I once just rested for three weeks but it was a minor limp on the front)