Do you mean he’s having a muscle spasm in the leg or that it gets stiff? I don’t know if it can be a sign of a knee injury, but when my male injured a muscle in his groin (illiopsoas) he did have muscle spasms in the area for the first few weeks of the healing process. The muscles in the leg would suddenly become tight and rigid and it was clearly painful for him, but would resolve in minutes.
If you mean that he is stiff when he gets up, that can be either sore muscles or degenerative changes in a joint, which could be from a knee injury.