My son is four and still has problems pooping in the potty. He goes #1 but when it comes to pooping he holds ?

I have tried everything from sitting him when he doesn't have to go offering diapers, pull-ups and even offering to let him go in his underwear. What should I do? When I ask why he doesn't want to go he said " a shark or alligator is going to come out of the hole". Please Help me I don't know what else to try.