I agree - Tell him until he can hit the toilette, that he must pee sitting down. And that if he can not do this, than he will continue to clean up his mess and you will start taking privelege's away. Don't think he has any.... Believe me they are there! Privelge's to take away: TV, Phone useage, early bedtime, computer, outside time, computer or video games, mp3 player, his favorite set of clothes, bike, skateboard, skates, extra events like a school skating party, overnights somewhere or even having someone over, no snack type foods, no soda, no chocolate milk....I can go on as I have had other issues with my now teenaged son and have done all of the above at one time or another.
And every time he does it, don't stop the previous privelege removal, add one to it as well as an extra day.
Also start adding chores to his list.... Have a dog - Make him pooper scoop the yard, cat - scoop the litter box, laundry, dishes, dusting....
Like you said, You also need to get to the root cause of why he is missing...And work on resolving it, even if it is a weight problem....