Stop, and get him somewhere safe and level
The first decision is to do less, not more. A horse that comes up suddenly lame, whether under saddle, on the yard or in from the field, should be stopped, dismounted if you are riding, and led at a steady walk by the shortest sensible route to a stable or a small pen with level footing. Do not trot him up repeatedly to see how bad it looks, do not turn him out hoping he walks it off, and do not give him a few days in a big field to test it, because each of those turns a diagnosis into a guess and a minor strain into something worse. Deep mud, steep slopes and being chased by field mates are the enemies of a leg that is already compromised, so small, quiet and close to the yard is the right first address.
Look before you treat: the two-minute check
Most sudden lameness in a working horse is in the foot, so start there. Pick up each foot in turn, clean the sole out properly, and look for a trapped stone, a nail or screw in the sole or frog, a shifted or sprung shoe, and a hot, tender heel. Then stand back and look at all four legs together for the one that does not match: a filled tendon, a swollen fetlock, a cut that seemed trivial, heat you can feel with the back of your hand. Walk him a few steps on hard, flat ground only if he is willing, and note honestly whether he nods his head as one front leg lands, shortens a hind step, or refuses to put a foot down at all. A bounding pulse in the pastern of the lame leg is another clue worth reporting, and it costs nothing to check.
Cold, rest and quiet: the first hours
If a leg is hot or swollen, cold is the useful first treatment: cold-hosing for a good long spell, or a cold pack held in place, repeated through the day, helps limit swelling while you decide what you are dealing with. A support bandage on both front or both hind legs, if you bandage confidently, keeps swelling down and protects the opposite leg that is now carrying extra weight; if you do not bandage well, leave it off rather than do it badly. Box rest with hay and water within easy reach is the default setting, not a punishment. Pick the feet out again later in the day, because a stone bruise or an abscess brewing in the foot often declares itself over hours rather than minutes.
The rechecks that tell the real story
What turns first aid into useful information is the pattern over the next hours. Is he bearing more weight by evening, or less? Is the swelling bigger, smaller, or moved? Is he eating, lying down normally, and passing manure? A lameness that melts away with rest points one way; one that deepens, or swaps legs, points another. Someone should look at him at set times rather than whenever they happen to pass, and each look should be written down, because 'about the same' means little at the far end of a phone call to the vet.
When lameness is a call today, not a wait-and-see
Phone the vet the same day if the horse will not bear weight on the leg, if there is a puncture anywhere near a joint or in the foot, if a leg is hot with rapidly spreading swelling, if he is sweating, trembling or distressed, or if the lameness comes with a temperature and a dull horse. Do the same if a mild lameness has not clearly improved after a short period of strict rest, or improves and then returns as soon as work resumes. Coordinating those timed rechecks through KinsLe's Care Teams means the checks actually happen, each one is marked done with its time attached, and the vet hears a clear sequence of change instead of a summary pieced together afterwards.