Unfortunately Fusion is too far in the future to solve climate change. There are still a bunch of hurdles we need to pass to start making feasible commercial reactors, so investment into other fields is necessary for the short term.
Darkrib
Red rising Trilogy by Pierce Brown is the first that comes to mind, it follows the young male protagonist in admittedly a darker and more mature story, but the action and story keep you very engaged. Fantastic character work as well in my opinion.
Another scifi series that's very fun but not necessarily an 18yo male is the Murderbot Diaries by Martha Wells. I've only read the first novella, but I'm constantly on the lookout for the rest I enjoyed it that much.
I think it really depends on what you mean by "fun", and how willing to stretch the 18yo male protagonist.
If fun just means on your toes excitement and generally being a good read, I can definitely second Wheel of Time. It's an older book, but reads like modern fantasy. Lightbringer is also a good one, though I haven't read the entire series.
If fun means more lighthearted or a positive outlook, that's not quite my expertise, but I have heard many good things about Discworld from many different people.
If you want to loosen the 18yo protagonist, lightning round! (There's a lot of Sanderson, but I do love his stories) Mistborn, by Sanderson: 18yo(ish) female protagonist, not your typical YA young girl though Stormlight Archive, by Sanderson: Multiple POVs, two maim ones being 18yo male and female. Great story, as others recommend. Kings of the Wyld, by Nicholas Eames: Follows an older protagonist, more of a mature tone, but it kept me hooked until the end.
I also have some scifi recommendations if you want those.
I think that we will reach net energy output within 30 years, with another 30 to actually scale that into a usable reactor, then another 30 to actually start using it widespread, and that's assuming that costs continue to decrease on things like superconductors, and we get our tritium supply chain to exist. I'm a little biased as a student going into the field, but I'm hopeful.