Been there! My son just FINALLY trained at the age of 4-1/2, and used to put up a big fuss too. It may help to back off for a couple weeks, but just casually read a "potty book" or two. My son also loved the "Elmo's Potty Time" video. This keeps the concept in their head, but without pushing them and causing them to put their backs up about it.
I was convinced my son would be going to Kindergarten in pullups, but an amazing thing happened - one day he announced he needed to go potty, he went, and since that moment has been completely toilet trained - even at night. Not one accident. So the good news is, your son WILL go when he's ready (even though it seems hopeless at the moment!)and is likely to be done with it. I think some kids seem to want to wait until they can do it perfectly...they seem to know when that is. Keep it positive, avoid power struggles, and try to make the idea of going potty fun - singing songs, silly rhymes, the lure of really cool big boy underwear, etc. Good luck!