Help my child won't stop soiling himself?

Hey Guys, I am at the end of my rope and thought some of you out there might have some insight. I have a 3 and a half year old son, a 2 year year old daughter and one on the way in October. I potty trained my eldest at 1.5 and he was magic even at night until just after Christmas. Then he started having 6/7 sometimes 8 pee accidents a day, and more recently started to poo in his pants and just SIT THERE wallowing in it. I have tried rewards, talking to him, EVERYThing and I am now crying over it on a daily basis. My 2-year old is almost ready to train but I am reluctant to train her until I have this sorted out because basically this problem affords me little time for anything else. Any suggestions?