Hi if you have a dog grooming salon around you i would pop in some time they sell shampoo and will tell you what is best, or go to the pet shop and get some puppy shampoo and add some tea tree in. make sure you get all the shampoo out and towel dry and if you can dry him with a hair dryer that will be better.
I work in a dog grooming salon and we have owners saying that they bath their dogs but they never smell nice for long, not like when they have been to the groomers. And its down to the drying of the dog. most people just towel dry you need to get all the water out of the dogs coat.