I think as long as you keep up with his excercise he will be fine. The neutering age coincides with maturity so they can become less active as they move out of the puppy stage. If you maintain his activity and switch to a good adult food when the time is right he should be just fine. None of my dogs ever gained weight after they were neutered. My Baby gets neutered a week from today
I am so nervous although I know he will be fine...