Great advice about the self flushing toilets.
Try getting the potty chair out again and see if he is afraid of the big toilet but the little one is OK.
You could also try one of the small padded kid seats that fit on the toilet seat. He may have fallen in and be afraid of falling in again.
Have him pee in a recycle plastic container and dump it in the toilet until this passes.
Ask him if he saw a ... in the bathroom. Use safe things like fire fighter or puppy, mix in a few scary things and maybe he will let you know what scares him. An older child could have said something to scare him. (My older brother told a younger sister that a rat would bit her bottom.)
My 2 year old grandson was potty trained as long as I took him often. If I didn't he would start to wet his pants and run for the bathroom leaving a wet path.
I now give him dry treats. He gets a treat if he is dry and then used the toilet. Some times he pees three times in a row but I still give him a dry treat then distract him onto something else.
His parents will come to pick him up and ask if he has to pee. He screams no and pees as soon as they get two blocks from my house. I have taught his parents to ask if he needs a dry treat. He will run in and pee for a dry treat.
No more accidents.