I got hefner at 12 weeks ad he was already neutered. Now, I wouldn't recommend that but I think 6-7 months is the best age because you want to make sure if he has retained any baby teeth you can have those pulled at the same time to avoid anesthesia twice.
I would take a tour of your vets operating room and make sure it is state of the art and that they have two people working on him, one to do the surgery and one to monitor his vitals and the anesthesia. Preferably two vets (it will cost more but is worth it) but a vet and an experienced tech would be okay. Some vets try to cut costs by doing it all themselves. Not on my baby!!!