That your Bullmastiff has been very well socialized is a must. Choose your dog breeder with care and do not hesitate to ask questions. With such a large, watchful dog with a protective drive, it is even more important than with other dogs that his experience as a puppy optimally prepares him to become a reliable four-legged friend. If the dog moves in with you, take over the further education.
Your young dog is smart and docile, but also very independent by nature and a little stubborn. Convince him gently, amicably and without stress and violence that it is fun to work with you in a team. The attached four-legged friend is very receptive to it. A lot of praise should show him what behavior you welcome with him. Consistently stopping behavior that he shouldn't even get used to shows him, on the other hand, that you are the boss and set the rules.
Spend a lot of time in the phase of getting your dog used to everything. Other dogs of any size, strange and well-known people, children, city and country: what your dog gets to know as a puppy will remain familiar to him throughout his life. Right from the start, the dog should be used to being stroked by strangers, or to get a command like “sit”.
In a dog with a protective instinct, good experiences with humans are also irreplaceable. Going to a dog school with the puppy is a good idea - especially if you still need guidance and support in one or the other matter.