If you have no idea about what this could be or how to go about treating it, then maybe your best bet would be to call your vet.
There are many things that this could be. I would start from the hoof up when there are no obvious signs of injury, swelling or heat anywhere else. Just some ideas, hoof abscess, sole bruise, bad case of thrush, has the horse been trimmed or shod recently? He could be trimmed too short, have a close nail, or been nail pricked,etc Could still be a leg issue though, minor strain or sprain in the tendons or ligiments, hock or stifle, or even a pulled or strained muscle in the hindquarters.
Some "tests" you can try if you haven't already are, hoof testers if you have them and know how, gently tapping over the nails with a hammer, check the digital pulse is it stronger on the lame leg than the others, run cold water over the legs and feet and if the unsound leg/foot still feels warm and dries faster than the sound one then there is heat there that you didn't notice before. Palpate the leg with weight on it and also while being held up, compare to the sound leg.
Also, I wouldn't recommend the liniment at this stage, if anything I would be cold hosing.