One thing that may help is putting hay in their litter box. It promotes digestion, so they are more likely to urinate in the box while they're eating. Plus, putting hay in their litter box will give them a reason to be inside it.
It's a good idea to reward your pig with a little treat every time you see him use the box. This will make him think "litter box=food". Food is a good motivator for piggies.
Good luck!