A very realistic possibility is damage to the suspensory or deep flexor. This does not always cause hugely-obvious lameness.
Another possibility is a jar to the joint (bruising the joint capsule), or a sprain.
If there is heat in the hoof, it is even not-impossible it could be from a hoof abscess, although usually there would also be a lot more swelling lower down, and lameness.
(Note that windpuffs, per se, are not painful. Although you can get a windpuff-y type enlargement of the joint capsule from a traumatic injury or infection that IS painful, generally windpuffs refers to a cold "set" painless chronic enlargement of the joint capsule)
Definitely your best bet would be to get a vet to look at him (preferably, if you can, a vet who is good at seeing lameness, not the type who prefers to rely on doing a lot of ultrasounds and xrays and all that as a substitute for a good eye).
However if the horse is just an unworked permanent pasture pet, and does not seem particularly unhappy, I suppose you could go with the "I don't know exactly what it is, but all the likeliest possibilities have the same general treatment plan" approach: As long as the horse does not appear lame, cold-hose the leg as many times a day as you can stand (3-5x is good) for 15 minutes at a time. If you know how to do a proper, supportive, racehorse-style standing wrap, then keep the leg wrapped; however if you are not very experienced at this, now is NOT the time to learn, and it will be ok without. Try to keep the horse quiet but mobile for the next month or so -- if he will restrict himself to the walk when left on 24/7 turnout, that is ideal, but if for whatever reason he is inclined to get to running around, that is a Bad Thing and needs to be prevented (by adding a quiet companion, by turnout only in a small pen during times when he is predictably excitable, or by stalling at night then handwalking for a little while before turnout). At this point, there is probably not a huge lot of point in considering Bute, since the injury is already a week and a half old. DO NOT MASSAGE a joint area containing potentially-injured tendons/ligaments!!! (Although if you can afford a normal body massage for him, it wouldn't hurt, although it is also not likely IMO to help hugely)
If (as I would be inclined to bet) it is a suspensory or deep flexor strain, you have a reasonable chance of him remaining pasture-sound, although likely with a permanent windpuff and the injury liable to recur.
Best of luck,
Pat