I would make sure the puppy knows your son is an extension of you. Back up your son on all things over the dog. Make sure the dog respects your son.
Having your son be respectful of the puppy, now that he knows there are teeth in that cute face, is not necessarily a bad thing - but you want to work with him to get over being afraid, too...
A trainer with fila (or at least mastiff) experience would be great if you can find one. Otherwise, being just 16 weeks old, you still should be in major socialization mode with the puppy - take your son and the puppy to new places every week (a new park, a place to swim, a strip mall, garden store, pet store, etc.), and introduce one new thing every day (umbrellas, people wearing hats, new noises - vacuum, blender, hair dryer, etc.). Then give the puppy a nice quiet place to nap and absorb all the new sights and smells he was exposed to.
Maybe you can enlist your son to help you brainstorm new places & things to introduce the puppy to... are/were there things your son was afraid of that he'd like to help the puppy learn about, so the puppy isn't afraid of them? Understanding dog communication styles, and that the puppy can also be afraid of stuff might help your son bond with the puppy, too.