If it's not too deep or needing stiches, just wash with warm, soapy water and rinse once or twice a day until it heals. You can dab a thin layer of Neosporin on it but if he can reach, he'll probably lick it off. Just keep an eye on it and if it gets pussy, take him to a vet. But it will probably heal up fine. I wouldn't use peroxide because that tends to inhibit the growth of new epithelial tissue, it damages regenerating cells. Soapy water is the best because it breaks down surface tension and allows for deeper cleaning. Hope he heals up soon.