Another Potty Training Question

So we did the 3 day potty training thing with my youngest. He is doing really great as far as peeing goes. No accidents since the initial training, tells us he needs to go, goes in public, etc... With pooping he will tell us he needs to go and does and then about 20 minutes later will finish in his undies or goes in his sleep? He does seem concerned about falling in the toilet but only with pooping? Any thoughts, ideas, suggestions??