Whatever he has been eating. I use Pedigree Puppy Complete Nutrition. If you want to change brands slowly make the transition to the new brand by gradually mixing them together. Here is an article to go by: http://www.drsfostersmith.com/pic/article.cfm?aid=99
Both Pedigree and Iams are very low quality foods with terrible, species-inappropriate ingredients. I would definitely try to find a tractor supply in your area.There aren't really any decent dog foods at Wal Mart, but I guess if that's all I had to choose from, I'd choose Iams. It's better than Old Roy, Pedigree, Kibbles 'n Bits, etc., for sure! Instead of supplementing his food, why not buy a better brand to start with? Do you have a Tractor Supply in your area? There are some decent foods there that won't break the bank. They have a store brand, called 4Health that's very reasonably priced.