I understand your concerns about your son.
You can talk to the school to let them know this issue that way they can help remind him. You can also try explaining to him that in school the other children do not do that and he needs to try to start remembering. Once he gets to school believe it or not it may hit him and he can realize that your not there. He will take it more serious. He may just be comfortable since he is home with you and your remind him, but when he is there and sees he does not have that luxury I bet he will get serious.
You can start now by letting him know you are going to try something new. You will not remind him. You can create a chart and reward him when he does it himself. This may give him the incentive to start doing it himself now before school starts. When he sees he will get stickers and in time get rewarded for doing it on his own this can help him remember and be motivated. The reward does not need to be a present. It can be a privilege at home as well. Little things mean a lot to children and those are the things that matter most.
I wish you all the best with this and in the long run it will all work out. Don't worry just take it day by day and I wish you all the best. Good Luck to him in starting school!