I understand if the sound of it annoys you, it would me. Have you read Honeypots thread Nail biting in Dogs... ?
If your dog doesn't fit the description of a dog that does it to clean their paws in a non-obsessive way, then there's a reason. And the list of the most likely reasons isn't that short, according to my books.
Itching due to fleas, skininfection, allergy or something else.
Injury, so small that it's not easily detected.
Pain, doesn't need to be in the paw, f. ex. if their back hurts, they can do it to release substances I can't translate the names of, and/or to simply take their mind of their pain.
Boredom, stress, anxiety or it could be an obsession, maybe due to once having had a problem with something of the things already mentioned.
The fact that you say he does it constantly, and that you don't think he enjoys it either, makes me give you a different answer than I gave on the other thread. So if you don't think you can solve it with more brainexercise, or reduce his stress, then do see if your vet can't find an explanation.
It may have originally started from a bacteria but if it's been going on for a long time it may have now become 'habit'. Is the skin inflammed? have you spoken to your vet about it, maybe worth having a skin scraping to rule out mites, or possibly shampoo with an anitfungal/antibacterial shampoo daily leaving the shampoo in contact for 10 mins before washing off.