K.D.
My daughter was the same way. She would poop on the potty but it was to much trouble for her to stop and pee so she would jsut go in her pants. I don't know if your home with him or not but my Pediatrician suggested using a 'Potty clock" and that worked for us. How it worked was we used the kitchen timer and set it for an hour(you can do it for longer to start if you want) and whne it went off it meant it was time to sit and try on the potty. After about a week or so she started going on her own!
Good luck!!