I don't think it's "he CAN'T open his mouth, " but rather he WON'T open his mouth. He's not going to eat until he feels safe in his new environment. Tortoises are territorial. He's been moved out of his territory into a new one and he's stressed. It takes a while for them to become comfortable and feel safe.
Another thing to consider is maybe you're offering food items that he doesn't recognize. Try to find out what the previous owner was feeding and offer some of that.
Another thing to consider is maybe you're offering food items that he doesn't recognize. Try to find out what the previous owner was feeding and offer some of that.