Honey Teddy Grahams or the cinnamon ones worked well with my kids. Offer him a few as a reward if he uses the potty instead of his underwear.
He might just want attention , soiling himself might be a cry for help , does he wear underwear ? a Lesson well learned is when he gets upset from soiling himself , that you let him sit in a area like that , until he understand thats its nasty and he needs to use the bathroom. Try making using the bathroom fun and easy for him , just like when he learned to urinate in the potty .