Why don't you try putting a piece of paper for him to go on in the pea gravel area til he gets used to the feel of the pea gravel. The more times he goes out there the more he will get used to it and, think of it this way, the paper isn't so bad to clean up. Just put some paper by the door and grab a sheet when you go out. Keep saying "go potty" as he is on the paper. Then try without the paper after awhile. He should get the idea.