It is interesting that you have given the list to the helper. Getting organised begins at home, and being disorganised is easy if you know someone else is going to take care of everything for you. How is he going to learn how to do it himself, so he can do it at school?
Model the behaviour you want to see, show your helper how you want her to teach him, create lists that he can use, and praise him for the things he remembers to pack. This type of support starts early.
Interaction while planning helps to stimulate your children's thinking and develops their ability to plan and follow a plan. This strengthens their ability to organise. Research on computer games suggests the ones that stimulate children's learning encourage them to think about what they need in advance.
If your son has not fully developed this thinking process, start working on it with him.
Pack bags together and ask questions such as, "Should we take water?" Add these phrases into your discussions so he starts to realise he needs to think. If he wants to take something he doesn't need, ask him, "Why do you think we need that?"