What should I do to get my month old puppy up to weight he was the runt of the litter?

I got a month old english mastiff mixed with a pitbull he is under weight I got him good eukanuba puppy chow to help get him the proper good stuff he need's to grow. He has a little fat belly after he eats but you can look at him and tell he's under weight because you can see his rib cage.