The idea that neutering will make him "lazy and fat" is an urban legend. I have had two males. Roman passed away at 11 a svelte 85 pounds of muscle. He loved to hike, swim and run. Cancer caught him, but nothing else could. The only thing slowing him down was the other end of the leash. Leo is 4. He's still all energy and there isn't an intact dog I personally know that has anything on his energy level or stamina.
Owners make dogs fat and lazy, then blame the surgery cuz they won't acknowledge they don't do anything with their dogs.
What age to do so, or wether or not to, is a personal choice. But please don't think that an altered male will automatically become an obese couch potato. It is simply silly.