I would bath him about once a month.
If the baby shampoo works for him and his skin then great. I have heard that it can dry out a dog's skin, but who knows. If his skin looks fine then no worries

I find that dogs with a short coat like boxers, dobermans, etc tend to get that doggy smell faster than other breeds. Their coats just seem to get more oily and attract dirt faster.

As a basic guidline I go by petting my dog. If I feel like I have to wash my hands after petting them....they get a bath!!! LOL