E.P.
We had this problem too! We never found a seat that fit correctly but a friend of mine suggested sitting him backwards on the toilet. It worked great because then he could hop up there instead of trying to back-up onto the seat and since the toilet seat was more narrow in the front it was a perfect fit for his little toosh. If you decide to try it just remember to have him lean slightly forward so the stream of pee goes into the toilet. Hope this helps!