I did something like that with one of my brothers who also refused to carry anything on day hikes or gear up for backpacking trips. For years I'd asked him to carry some basic gear and he'd always say "I won't need it" so I'd carry extra clothes, food, and water.

Finally I decided that I wouldn't do that anymore. We went out on a day hike, he got cold, and ornery, and even took off and left me behind while he raced back to the car at the trailhead. I had the keys to the car in my pocket and I was warm and fine so I just watched as he took off ahead of me.

He was shivering cold when I got back and would hardly talk to me. To this day he thinks I was a selfish jerk that day. He still doesn't consider the dozens of times I packed gear to cover his ass, and he used it. He doesn't consider the even more times I packed it and he didn't. All he remembers about that trip is it got cold and I didn't offer my clothes even though I had several layers on. We haven't been hiking together since, and I still feel a little guilty about how the day turned out.

With quite a few years to reflect on it I think that if I'd just insisted he carry the gear I knew he might need he would have and that would have been a better approach to deal with the problem.

On the other hand, had I done nothing he would have gladly let me carry gear for him on every trip since then, so I'm not entirely unhappy with the outcome.