A list - you need to give him a list. You shouldnt have to, but see the consequences when you dont!
I have a list for DS's swimming bag just in case I need him to pack it, cos I KNOW DH wont think of everything.
He might manage a swim nappy, but he wont remember a cossie as well (not the end of the world as its the nappy that matters)
He definately won't remember to bring a normal nappy to put on DS afterwards. Its unlikely he'll remember more than one towel and its unheard of that he'd remember to pack my/his cossie too.
Its the same with DS's everyday bag. If DH takes him out on a weekend, I have to oversee the bag packing, otherwise I get 'He (DS) was really whingey because he didnt have any snacks/dummy/wipes/nappy/juice' and so on. Well, thats because the fairies dont magically put it in there and daddy really should know what is required by now as DS is nearly 2 and a half.
But I am thankful he takes him out at all so its no big thing to oversee bag packing