Potty Training

We're halfway through week 3 of potty training Nathan. He's done really well going pee-pee in the toilet and rarely has accidents. But does anyone have any great tips for getting him to go #2 on the TOILET? Not only is he pooping in his underwear two or three unpredictable times per day, but now he won't tell me when he goes (probably because I will show my disappointment), and he'll SIT in it, squishing it into the fibers of his tighty-whities. Not fun to wash out. Not fun at all.