I think it would be appropriate to make him a checklist (then have it laminated where he can use a dry erase marker every time). Then, you just need to say, "Check your list!". After this, then yes, I think he should be responsible for his things. My son recently forgot his cup for sparring at karate testing. I specifically asked him before we left if he had it and he said he did, but did not check. Unfortunately (or fortunately), he had to use HIS money to buy a new cup on the spot at the dojo. Nice lesson for him. Now he checks his bag. He's 11y/o.