WISH I could help more! My son just turned 4 in November and is STILL not potty trained. He still sometimes does the peeing in the corner or in drawers and tries to hide it even though of course he knows it is wrong. We've tried everything from the cheerios in the potty to going every hour to my completely not mentioning potty at all to see if he just goes on his own. Nothing works with my son.
I do find he does better when he is not in pull ups though. Pull ups work so good that I think kids who have difficulty using the potty just figure they'll pee in their pants because they know it won't make a mess.
Now how in the world did you get your son to poop on the potty by 18 months!