Have you tried canned dog food? It is better for them anyway because of the moisture and he might like that. I would say if he is eating a lot more than what the package says and still isn't gaining he needs to see the vet.
Some good brands of canned food are Evangers and Canidae but there are lots more. Iams/Eukanuba does very cruel animal testing so I won't support them. Their food is also not very good (don't tell your vet that though...they usually don't know too much about nutrition
). Purina is also not very good. There are lots of threads here about good food. Please remember to brush his teeth. Some people think that eating canned food leads to tooth decay. I really don't think that's true but it is healthier to brush their teeth anyway.