Like one of the previous posters said, a shiny coat comes from the inside out. Make sure that he is getting good quality feed, is up to date on his de-worming, and don't shampoo him too often. Any kind of soap will strip away essential oils and make the hair brittle. Lots of brushing and plenty of a good feed will do wonders.
I have to say if you're looking for shampoo, Cowboy Magic Rosewater. It is a dream. I just washed Lakota with it 2 days ago and he was so shiny and soft. I really like it because it got his white so white it was blinding but it also got his brown nice and clean and shiny. Plus, its hard to find a shampoo which leaves the horses shiny and soft because we use cold well water but Cowboy Magic worked like a dream. And he smelled sooooo good lol.