Lots of kids, even older than your son, have trouble staying organized and remembering the things they need to take with them. I still have trouble with this, and I'm in my 60's. But we come up with strategies of our own when we realize it's the only thing that's going to work.
I recommend you do one of the wonderful strategies taught in How to Talk So Kids Will Listen, and Listen So Kids Will Talk, by Faber and Mazlish. Get your son on the problem-solving team. Sit down with him when neither of you is in a hurry, and ask him to go through his morning routine. You write down the main points that he needs to remember, and help him make a checklist that he can hang on his wall and run through before leaving in the morning. He can even find pictures from magazines to stick on his checklist to make the reading faster.
Partway through his morning, ask him if he's following his list. Ask him again, at least 5 minutes before it's time to leave the house, whether he's checked his list and put everything in his backpack.
He'll almost certainly need some practice to stop forgetting things, and he may need to fine-tune his checklist to include occasional items like a toy for show and tell. But you'll be helping him tremendously in two ways: You'll be giving him a real tool that will help him take more responsibility for remembering necessary details, and you'll gradually be shifting the responsibility for the consequences of forgetting onto his shoulders.
I recommend you go to the source for this techniques and many others that will make your daily family life smoother and happier. Get the book. You'll wonder how you got along without it.