Have Grandmom ignore him, no reaching towards him to pet. That growl is a warning, listen to it. He is approaching Grandmom, let him initiate contact. Once he has made it to sniffing, have Grandmom hold a stinky treat, again not offering it, but allowing him to take it from her hand.
Again, Grandmom can not reach for him to pet or scratch. He has to relearn that good things come from Grandmom.