J.S.
I have three kids, and my only son seemed to be the one afraid of the toilet. Not only did he prefer a diaper to underwear but he hated going in the bathroom especially if they had auto flush. I used to forget in the beginning and would take him into a stall fighting the entire way and the toilet would go off. He would stop mid fight and climb my let, I could not hold back the immediate laughter (poor guy).
Over time I allowed him to push the buttons on the auto flush even if he was not the one that had to use the toilet. It still scared him at first but then he started asking me if he could do it and looking for auto flush toilets. We all fear things we don't understand so maybe if you can help him understand the purpose it will help. Bring him in the public bathroom even if you do not need to use it. Walk in and just walk around or let him look at himself in the mirror and wash his hands. You could also give him a special potty book that he gets to read only when using the potty outside of home (small enough to keep in your purse). Hope that helps, good luck!