Have you tried to bathe his paw with a saline solution? The reason I ask is that Vegas sliced his pad and it took ages to heal up. He did limp for quite a bit - even a small bit of gravel stuck in the cut was enough to make it painful. The vet advised me to bathe his paw after every walk. He was also given antibiotics and pain relief as his was a bad one. If Sweep hasn't perked up tomorrow, maybe give your vet a call to see what they think.