How do you get a 6 year old boy from stop peeing and pooping in his pants?

So I have a step son...and if he is on the computer or on the PlayStation and don't want to get off he just pees and poops in his pants...I truer grounding him...washing out his underwear..but he does it like nothing...Then he dose it all over again the next day...He is fixing to be in 1st grade...what can I do to make him stop?????