Take him to the vet. For the past nearly 3 weeks I have been pulling my hair out to get an incision on Orick's right hind paw healed where the vet removed a tumor (benign, but growing). She sutured it with the dissolving sutures, and within 5 days he popped it wide open. It was on the side of his foot, but the lower part went into the large pad. He also got it infected, and I started him on penicillin. The vet put staples in it after he opened it (bounding down 5 steps and spinning at the bottom). All this time I kept it covered, but not airtight (finally found a use for all those hospital no-slip socks my sister has been accumulating). Also had to wrap it in plastic when I took him out (leashed) so it didn't get wet. Along with the oral penicillin, I put Neosporin on it at least 2x day. The big thing with him was keeping him quiet enough that he didn't rip it open again, and I mean by running/jumping/spinning--I put a cone on him inside when I couldn't supervise him.
The staples are out now, and it is healing, but it has not been a fun ride. Just warning you ahead of time, lol. If his cut is deep, he needs the vet, and you will have to be persistent and diligent in getting it to heal!