T.S.
Hi J.,
I agree that it could be the plastic or what you are using to clean the seat. Children's skin is typically much more sensitive than adults. Many plastics are made with ingredients/chemicals that can be very irritating, such as phthalates. He may also have an allergy to latex, which could be found in there too.
I would try covering the seat with a different material or getting a different seat all together. Also, try using safer alternatives to clean it with, like just simple soap and water. Many of our everyday personal care items and household cleaners contain irritating or harmful chemicals as well. I could recommend some safer alternatives, if you like.
Good luck!
Sincerely, T. S.