hi K.,
I definitly agree with all the other mommies about getting big kid underwear. As far as well, pooping on the potty, the first thing to do is to make sure he is not constipated. If it hurts to go, he won't want to do it. Next step is to put him on a potty seat when you know he is "due to go" WITH A DIAPER ON! He may be feeling possesive over his you-know-what and not want to part with it, or the sound of the splashing may scare him. Read a book, sing a song, etc. When he starts to go on the potty with the diaper on, and is comfortable with this, the next step is this: when he is due to go, take a diaper when he is out of the room and cut a hole in the bottom. put this one on him and set him on the potty and let him go that way. He will still feel secure with having the diaper on and should be able to go. definitly still use a little potty for this-the whole splashing thing. Also, if he doesn't seem to want to see his uh, production, don't show him. My daughter was scared to look at hers! After a while, you will probably be able to have him go potty sans diaper!
I hope this helps you-I got it from my pediatrician, and had to do it with my son. He was potty trained at 17 months-number two included! :)