Evee

My (long-haired) dachshund Frank keeps licking his back legs, like none stop until they are all wet and nasty. sometimes he bites his legs too and his nails! I don't know what to do about this. I asked his vet if it was mites or flees and it isn't and i don't think it's mange (how would I know if it was, i've checked sites and they talk about what it is and try to see stuff, is it curable?). I bathe him once every 2 and a half to three weeks with a mild shampoo, i clip his nails when they get long enough (when they touch the ground when he's standing still) and i groom and comb him often- I don't know what to do!?! he doesn't whine while he does it so i don't think he's hurting himself, but it can't be good for him. Oh and he does it at night, before bed. HELP ANYONE PLEASE!!!