How tall is he (at the withers)? It is hard to tell from the pictures, but he does not look too skinny to me. He will fill out over time.
However, Purina Dog Chow is not a good choice and you should feed something better. A good place to start is Dog Food Reviews and Ratings | Dog Food Advisor
. Look for the five and four star foods. Any of these will be a great improvement over what you are feeding now.