Do some research and show him. Remember dogs age a lot faster then humans. So, just like humans, they can get testicular cancer, prostate problems and/or cancer. They should get their prostate checked everytime they come in to the vet, which is hopefully anually... it's not a comfortable thing to go through as a human let alone having it done by some rando that doesn't speak your language. Also the dog is more likely to get into trouble because he can smell a female in heat a mile away or so. Some intact males will stop at nothing to get to the female. If they can't they get frustrated and stressed. So if your dad really loves his dog then why would he let it go through all the crappy stuff of having testicles? Neutered males (and spayed females) tend to live longer too. I hope this makes sense sorry it's so long. I work as a Veterinarian Technician and see way to much shit. Good luck!