I agree with
@boskelli1571 possible sprain or damaged ligament. I would give him several days to recover. Chickens can slip/slide of slick surfaces and I wonder if he landed doing a split or something like that? It's hard to know.
Does he appear to be in pain? You can offer aspirin at 81mg per quart of water (1 low dose Bayer) I use the orange flavor along with a little honey to sweeten the water - aspirin is little bitter, but I've found mine accept a little "sweetened" water

I would only give the water for a couple of days. See that he is eating/drinking well. Poultry vitamins and a little protein to give him a boost too.
Also, if you can, cage him next to his hens, he will be calmer (usually).