Bike Baseball Pants

My step son has continued to soil his pants with his feces since he was 4 years old. He only did it here and there at 4 but by 7 he did it all the time. He lives with his birth mom who won't do anything except make excuses for him. He hides it from us, he has no problem eating breakfast, lunch, and dinner w/ feces in his pants. Has no problem going to sleep with feces in his pants. Does it at school, does it at baseball games. Acts like he doesn't care. We have done it all: from punishing him normal, to yelling, to time out from ps2, bikes, DVDs, TV, etc. We have purchased the big kid diapers and make him wear them. We have been to psychologist, gastrologist, etc. They tell us he is highly constipated and just needs extra bathroom time. He tells us he is too busy playing and just waits too long. He once pooped his pants at a wedding sitting right beside me rather than telling us he had to go to the bathroom. Please help, PLEASE....we don't know what else to try!!!!