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As long as the ribs don't show too much, he doesn't have a problem. With most Labs, over eating is the problem,
I did have a male Lab puppy once that refused to eat the regular adult Purina when I switched him to it. Switching him to a concentrated adult chow similar to Iam's kept him fit to take out in public. I think Iam's is a little high in fat, but I know of a couple of service dog schools that feed it, and nobody knows more, or is more interested in long, active lives as them. 1-2 cups sounds a little low to me. I think by 4 months, my Labs are usually eating at least 3 cups.
If your boy fails to eat enough to maintain the ideal body condition on adult Iam's, take him back to the vet. If that vet doesn't find the problem, try another one.