If he's only eating apple, he's not getting a proper diet which could be helping the diarrhea. Is he dehydrated? (does his not move back in to place after a second or two on him if you move it)
If he is a picky eater, try blending things up. That way, he smells and tastes the apple in there, but is getting other nutrients as well.
My advice is to avoid acidic foods right now too, they can cause diarrhea.
Does he have a staple diet, if not; the introduction of a good staple might help.