He really does NOT need to gain weight. His weight is fine. However, if you choose to up his weight a bit, my guess is that a good pet weight for your dog is about 80 pounds.
Dogs in general do NOT need to be "Labbie" fat. The majority of pet dogs are overweight, usually grossly so.
Keep in mind that the amount of food suggested by a dog food company is just that, a suggestion. Every dog has different needs and will need a different amount of food. Your Vet is also making a suggestion.
I have a big male who did not race. When I brought him home, he was about in the low 70's. I kept him about 77 to 80 lbs and he looked good. I noticed that he had tanked up again and is about in the mid 80's now. He will be going on a diet to drop some weight.
My dios eat 1 to 1.5 cups of Purina One lamb and rice once a day. They get a treat when they come inside from a potty trip on the yard. No one is starving in our home. Now if you ask the dogs if they are, of course they will say I starve them, brats!