My 3 yr old son will not poop in the potty. Help?

I have a 3 1/2 year old son. He willnot poop on the potty. I have tried the 3 Day Potty Training system which worked great for the pee but he will not tell me he has to poop until he has already gone in his underwear. He is not afraind the potty, bribery won't work, he is not scared to go in it. I am losing my mond with this. He his ready I just can't get him that last final step. Please help...anyone