Don't let him eat when you're working with him! Be very consistent and only release pressure or reward when he does something good! And ride with a whip or spurs. Ask him to go by clucking, then with your legs, and if he still doesn't go tap his but with the whip or roll your spur across his belly. DONT KICK. It will desensitize him to his leg. Also lunge him and use vocal cues and make sure he understands them :)
Try getting a dressage whip or a crop to tap him on the butt. Do ensure that when you bring him in you feed him so he understands coming in = better food.
If that doesn't work, try putting a bridle on him so you have better control.