I don’t think this can be ‘cured’... now that he knows what he can do, you need to fix this before something worse happens.
then how would you feel?
Its out of your hands
Most dogs, even old dogs can be trained, but this is a big animal.
Best to train them from puppies.
Any aggression toward a child should be dealt with immediately.
Face nipping should be nipped in the bud.
It is an animal instinct that they go for the face or neck when in alpha mode.
In pack mode they will tend to legs or arms if YOU are in an alpha mode, unless YOU correct them.