Well first off welcome. I don't really know what you could do for his oily skin. I know dish soap would dry it out, which is what he might need, but I don't know if you should use dish soap every week. Maybe just when he is extremely oily. I know alot of users use: MINK SHEEN - 4 in 1 Pet Shampoo,
Mink Sheen 4 in 1 Pet Shampoo, but its a gentle shampoo and it sounds like he might need one with more, umph. Good luck with finding one you like. I'm sure someone will be able to give you some good advice.