Hi there, thanks for writing in.
There are several possibilities as to what this could be. Crusty scabs sounds like it may be a superficial skin infection. Sometimes they develop around the lips because of excessive moisture. Being that he's a great Pyrenees, he may be slobbery which can alter the normal skin barrier's and predispose him to skin infections.
You also can't rule out mange or even ringworm. Again difficult to determine just based on your description.
I would suggest that you bring him to your vet tomorrow to be evaluated. He may put him on some antibiotics for the skin infection and can give you a medicated shampoo to use locally on the affected areas.
I hope this has helped. Thanks for your question.