Does he has his own little potty? If yes, make sure it's a solid one because if he feels by sitting on it he will fall side ways he will not go. Big toilet like we use scares children. They are scare they are going to be swallow up by it.

Another thing, when you go yourself, bring him with you. Show him it's a normal thing to do and that you can even looks at books in there. Make it an amusing thing to do. You can go as far as buying a special soap, something that smell very good, that only, if you went on the potty, can you use it.

This is only suggestions. Good luck!