The less discussion the better. Let the reward system do the talking, if you will. Do not take back the rewards he has earned. That will turn the reward system into a punishment, just what you want to avoid.
So when he does have accidents should we discuss it with him? The reward system we were thinking about is using cheap hot wheels. Everyday he goes a full day without an accident we would give him a car and if he does have an accident he has to give all the cars back? Is making him give the cars back a to negative to add to the reward system or should he have only rewards when he does good, no consequences for mistakes?
First, maintain your equanimity - you're letting yourself get way too upset. This is a normal routine. Your son is on his way to being completely toilet trained - he's just not there yet. Absolutley use a reward system and do not become angry. The nocturnal enuresis may be a separate issue altogether. Roughly 8-10% of children are wet at night, even if they are toilet trained during the day. The two things do not necessarily go together.