what to do?

My son Chris has a problem soiling his pants. He is seven, and has started having this problem shortly after we moved at the beginning of last summer. I don't understand why, and am running out of things to do about it.  Half the problem is, I don't have alot of time to do anything about it. I see the kids - rush them rather, from when we get up at 6:30 to 7:30 when I drop them off at school, during which I am also getting them bathed, dressed, fed, cooperative, and for the boys, organized. Then from 2:15 to 2:30, while I pick them up from school and drive them to the parks and rec where they have afterschool (no school bus for anyone living under 2 miles to the school).then again from 6:15 when I get home until 9, when they go to bed, M - F. Home is not a sanctuary for me, there is chaos that needs constant intervention; it is the epitath of maxwell's law, so my time in the evening with them, exhausted as I am, is cooking, monitering the multitude of little friends that walk in the door to play, homework nightmares, the pull of their video games (my personal demon - T.V. screens are irrisistable, which is why I make no effort to get airwaves).  Star charts haven't helped. Christopher could hardly care less about them, reward or no. The pediatrician suggested that some boys don't like sitting on the toilet, and that therefor he must everytime he goes. It hasn't had much results, working in the times that I've had him. I've tried ignoring him when he's stinky - but no one else will. I've yelled, threatened, spanked, given extra work, bribed, you name it, no results. Maybe its just Christopher, he is twice as stubborn as a mule, carefree and happy go lucky. Unbound by natural law. Unfortunately, his problem is hurting our family. If he poops in his pants one more time at the parks and rec, he is going to be kicked out. And with no one I trust or is willing to watch him on weekdays (I live in a bad neighborhood), either I've got to quit my job or leave him at home unsupervised except by his older brother and sister, who are too young to be unsupervised themselves. What can I do?