If nobody else claims him, I'd keep him. He's a pretty rooster. Quarantine him for two weeks to 30 days first and check him for mites. You should go around asking all your neighbors if they lost him. He shouldn't hurt your girls, but if he's carrying disease, that'd be a problem. He looks fairly young. I'd say he's about six to seven months old. Someone might have abandoned him when he started crowing too much and figured you'd take him because you have chickens. (This has happened to me more than once. Thankfully the first time the people asked.)
As far as breed, my first thought is Blue Andalusian, though, I feel like he should be darker. His shoulders are pretty dark, so maybe he still is?