It could be a number of things! As you say, it might just be a simple strain, in which it will get better on its own.
Try and check his foot to see if he has a cut on one of his pads, or something stuck in one as this would make his foot sore and he'd be reluctant to take weight on it.
Another possibility is a bite wound abscess somewhere in the leg - if your cat is an outdoor cat the chances are he'll get into scrapes, even if neutered (although he's much less likely to do this if he *is* neutered) - but if you can't feel any lumps or hot, painful areas then the chances are it's not an abscess.
Worse case scenario, he's had a run in with a car - I've had cats brought in before because they were limping on a back leg, only to discover a pelvic or limb fracture.
If your cat doesn't get any better, seems to be taking a long time to get back to normal, or gets worse, I'd advocate taking him to the vet.