My son was hard to train also, his therapist gave me a flip chart with a potty story and also a picture guide of the steps of what to do every time you go potty to put on the wall beside his potty. It still took longer than my other children but it seemed to help alot. Goodluck!!!
When my daughter was being potty trained it took a lot of patience. What ended up working was to put her on the toilet every two hours day and night. It took about 3 weeks and serious sleep ddeprivation but it ended up working.
My son is now 22, and I remember when he was your sons age and NEVER thinking he would be fully potty trained, but let me assure the the day will come when he is. Consistency (routine)helps, having him sit on the potty frequently and regard him for when he's successful! When my son was in pre-school they insisted on cloth training underpants with plastic ones on top. She said it's important for them to "feel wet". It was a big mess, but it made sense. He continued to have some accidents, but they got fewer and fewer, so hang in there, I assure you the day will come when he is potty trained!