my 3 yr old son will not poop in the potty

I went through this with my daughter. She was great about peeing but just wouldn't sit down long enough to poop. We finally broke down and started offering candy as a reward. She got a lollipop (the small Dum-Dums) every time she pooped on the toilet. It only took her one day to figure it out and she's been pooping in the toilet ever since.

If your son seems legitimately scared of sitting down make things more comfortable for him. Make sure he's got a nice potty seat that he likes and make sure it doesn't slide around. Get him a step stool that he can put his feet on so he won't be worried about falling. Some kids even feel more secure sitting backwards facing the tank. Also have him present when you use the toilet so he can see that you don't fall in and make flushing seem fun and exciting. Have him say "bye-bye" to the pee and the poop.