I think you need to go to a registered breeder who will help you with the puppy and give you a well socialised pup.
I would not be desexing the dog until at least 8-12 months for a bitch and 2 years for a dog due to bone growth. Aggression is due to environment and genetics, not due to being undesexed or that you have not shoved the pup under the nose of every human/dog you have come across. I had an entire male Dogue de Bordeaux who was a registered therapy dog for the disabled and passed with flying colours even with other dogs lunging at him. So no, it's nothing to do with hormones.Positive reinforcement is the best way and just be sure to never ever hit your dog (one way to ensure your dog doesn't end up aggressive). You will also need to start socialisation ASAP, with as many different dogs (healthy vaccinated dogs though) and people as possible. It is important to start at this age to ensure the dog doesn't become aggressive.
Get yourself into a dog SCHOOL not one of those puppy preschool malarkys run at the vet. By people who know about large powerful breeds and how to train them. As for positive only, headcollars are not suitable for mastiffs due to their restricting the ability to breath and pressure on the neck. As for harnesses ... even the no pull ones will tow you. Training and a martingale from little and martingale/correction chain as they're growing then down to a flat collar if you like or stick with the correction chain. It's even what the breeders recommend. You're buying a powerful dog with unknown heritage and temperament, you have to teach the dog right from wrong early and pull it up on any BS it pulls on you (yes, even with the goo goo eyes lol)
try leerburg.com on puppy raising as well.