I'm not an expert, but I would walk him on a leash in the grass until he does his business...no matter how long it takes. Take him out every hour on the hour. You have to over do it. If and when he starts to go on the deck, grab him or pull hi and put him in the grass. You have to get the point across to him that it's not cool to go on the deck. He'll understand after a few good tugs on the leash.