I have never heard of “neuter dog food”. If your dog is doing well on his current diet and getting adequate exercise, at least a 2 mile brisk walk 5 days a week. He shouldn’t need diet food.
However, some neutered dogs do tend to put on weight, so watch the amounts you are giving him. Only feed once or twice a day, take it easy on treats, no fattening treats.
Labs can have cruciate ligament injury, especially if they are overweight.
Consider homemade or a combo with grain free kibble. I like Nutrisca salmon and chickpea.
Is your dog overweight now? That would explain the vet’s concern.
See General Guidelines for tips: http://www.homeovet.net/dynamic/php/downloads/dog-c8470f2c75dbe4b683205c3919ee2310/dog_diet_complete.pdf
