Thanks for the additional information. I really appreciate it.
It sounds like he could be brewing a lick granuloma on that paw, possibly as the result of some sort of irritation.
If this is a lick granuloma (a sore caused by licking), then the best thing to do is to encourage him NOT to lick.
Of course, I understand that this is easier said than done...but I do have a couple suggestions:
1. Bitter Apple/Bitter Orange. This is a spray that you can buy at most pet stores. It is safe to use on the body of a dog, and tastes horrible. I've used it with some success with my rescue dogs that chew at their feet or lick at a leg until it bleeds. The only downside is that if the taste doesn't deter the dog, then it does you no good.
2. If he's licking mainly at night when you aren't supervising him, then covering the area and protecting it from his tongue can help. With some of the dogs I've cared for, I've used a regular sock of mine and placed it on the leg. Then I tape the top to the leg with PAPER tape (make sure it's PAPER tape...3M makes a good one....any other tape will cut off circulation and/or cause a bigger problem because it's not porous and doesn't breathe).
3. Finally, you could try a little Benedryl at the dose of 1-2 mg per pound of body weight (meaning if your dog weighs 20 pounds, you can give 20 mg the first dose, and if that doesn't help, you can give 40 mg at the next dose) one time every 8-12 hours. If he's licking because something is bothering him, this might be just the trick to take the edge off and give him some relief.
If it doesn't get better in a day or two, you may want to have your vet take a peek, just in case it needs some antibiotics to get it cleared up.
I hope this helps!!