With our eldest (7 months old) he was trained to "leave" from an early age. He'll even leave roast chicken if we tell him to.
When he started to eat poo, he soon stopped with 'leave' ( tho' he will still roll in goose droppings, yukk).
The leave command was part of a progressive training program using a 'clicker'.
My advice would be to go to obedience classes and work from there.
Failing that, don't let him lick your face
