How should I approach potty training with my 2 1/2yr old son?

Hello, I am trying to potty train my 2 1/2yr old son. He understands what it means to use the potty & says he wants to be a big boy & use it. He knows when he needs to use the bathroom & will tell me either right before or right after. For the past few days he has been wearing big boy underwear & going to the potty to pee, but he will just poop in his undies. Twice this evening he has pooped in his undies.....a friend of mine told me to punish him for pooping in his undies and that way he would know it was bad....what do you recommend? What potty training techniques have you had luck with for little boys his age?