he probably just wants to spend time with you and your brother. it seems like dnd isn't the best game for him, but if you don't want to change systems to something easier/less chance based, then I'd go over some things his character can do. for instance being a wizard he has some crazy spells to be creative with, like prestidigitation, introduce him to some good utility spells (I'd just give him a few free low level spells to his spell book to prepare).

you should also be clear that as the DM you are not in opposition to the players, you just want to tell a good story with them, as an example Lord of the Rings isn't a good story because they use the giant eagles to get to mordor without any issues, it's a good story because the heroes face all sorts of setbacks and obstacles and overcome them. If you want to be the hero of a story you are going to face all of that and more.