Slightly under weight dog who won't gain more? HELP!?

my dog is weighing in at 64.2 and he's supposed to be 75lbs. he gets the exact portion he's supposed to recommended by the vet but won't eat the full portion, he's eating natural balance sweet potato and fish both wet and dry foods. he's on the correct amount for his size, he just won't gain or eat any more, we've tried small meals throughout the day, more food, more treats, he just won't, would raw food be a good option? I'm seriously stuck!! please only helpful comments no hate! thank you :)