I sympathize with him! I hate summer heat and bright sunlight. 🙂 Also, I'm lazy.
The vast majority of my hiking and camping is in the spring and fall. It's less crowded, has fewer bugs, is less hot, etc. Of course, different people, (especially of different genders, turns out,) can have much different temperature preferences. Experiment with different times of the year, and, most of all, communicate with each other to find the best sweet spots for both of you.
If the routes you've tried with him are too much for his enjoyment, try to find shorter hikes with more payoff. It's possible you can build up from there if it catches on with him. Also, are there particular goals or destinations that are particularly appealing to him? I love the alpine zone; I've had friends who love waterfalls; even the metal surveying markers on top of mountains can be motivators for some people.
Focusing on the positive in any part of life is a broader question. It's a mental habit, and people can learn it, but that's much easier said than done. If I knew how to teach it reliably, I'd probably be a wealthy motivational speaker. 😉