Keep working with him, daily. It's the advice we've gotten over the years and that we now give to owners of any breed. Keep working with them daily.
Someone mentioned getting some professional training. Not bad advice. If there is a dog training facility in your area, check them out. Agility, obedience, even scent work. We do a bunch of dog sports with our dogs, whippets now but previously those and mixed breeds. Ribbons and awards all over the place. All dogs benefit from activity and having a "job". Tired dogs act right more often too.
Some of the training classes we've attended over the years, agility if I recall, had some AmStafs in them. One lady bred them and her dogs were excellent in the ring. They are generally highly trainable.
Spend as much time as you can with him, but the standard advice with any large powerful breed it to be aware at all times. Especially around children, strangers, and other animals. And even yourself.
Good luck with the pup.
Geno
