This is a tough one. The thing he has to realize is that his backup plan is you giving him your gear and that's really a sucky backup plan.

He'll deny that, but he doesn't have any other, so that's it.

I'd put together a pack with some essentials and insist he carry it. Tell him you won't hike with him if he doesn't carry it, and you'll buy him a beer if he doesn't use anything in it, but it will cost him a beer if he does, and put two beers in the pack you give him