How can I help my toddler with toilet training when he gags every time he sees his poop?

My toddler is having a real hard time with toilet training. He almost throws up every time he sees his poop. He gags and will not go on the toilet. We have tried a toilet chair and ring that goes on the big-boy toilet, but he refuses to use it. When he sits on the toilet, he just sits there forever and only will pee. He holds the poop in until he get up and then will poop in his diaper. I dump his diaper in the toilet and let him know that it is where it goes to try to encourage him. How can I help him get over this fear and passed the stress?