Here's a thought, try putting cheerios into the toilet, just a few, and ask him to see if he can sink the little ships, at least when peeing. Then he can be rewarded for winning a battle! As for the pooping, I'm not sure what to tell you, maybe the same idea might work for you. Praise him as much as possible for doing such a great job! Try a sticker reward chart and for every 10 stickers he earns, he gets a special treat. Good Luck, I'm going through the same issues right now with my 3 yr old.
He has a little frog toilet cause he loves frogs and we tell him if he goes he will get to put a nickle in his piggy bank cause he loves to put money in his bank. I will try asking him what is scary too. Thanks.
Have you tried the reward system? Like if he goes find a treat that he really likes and save it for when he uses the potty.
You should probably also get him a little potty, if he doesn't already have one.
One last question for you. Have you ever asked him what he is scared of, about the pottyk? Maybe uf you knew what he was scared of you could talk to him about it.