T.
Try having him help clean up the mess when he has an accident. If my kids pooped in their underwear (NO pULL-UPS) they had to help clean up the mess. Usually, that just meant helping me dump the poop in the potty. Once they saw how yucky it was to clean up, they started going in the potty.
My son will be 5 yrs next wk and my dgt will be 3 yrs next month. My dgt has been potty trained since the beginning of August. My son did not get completely trained until about a month after his 3rd birthday. With both kids, I just threw away the pull-ups and used panties. For the first few days, i put the vinyl training pants over the underwear just in case they had an accident. They did not war pull-ups at night either. They had both been dry in the morning for a long time so I was not worried about nightime accidents. My son was completely potty trained in a week and my dgt in about 10-14 days.
Good luck.