If he can do all that, he's never really in any danger and there is no challenge. It’s more impressive if he’s doing this without any magic.
This is even a fantasy setting? This could be a modern or sci-fi setting, and if that’s the case he’s probably going to be a survivalist type with intimate knowledge of traps and most likely have a crap load of guns and his own bunker.
It’s a cool idea, but if he’s a hermit he’s most likely not going to care about making peace so much as he’ll want to be left alone. Hermits also tend to have bad social skills, let alone experts in diplomacy. Just trying to survive the war to its end seems like a good premise. I'm not sure about being "born" a hermit. Unless his mom was pregnant when she decided to abandon society. I think he should've just got sick of society and just left it. (You can make up the reason)
I’ll suggest the following events, outcomes and possible branches.
Invasion of homeland. Obviously successful to some degree due to the invading soldiers being there, but there’s probably going to be some resistance by others at first. The opposing side naturally is going to use various means to squash dissent. This could go on for years before the situation is resolved.
The Hermit can flee altogether which might involve an adventure in trying to find a new suitable home. Of course with a war going on, he’s probably going to have deal with it as he travels out of location of conflict.
The Hermit can stay and fight. To increase his odds, he might help the “homeland resistance” After all they’re more likely to leave him be if they win. Of course getting caught up in the resistance opens an opportunity for the hermit to possibly become less “hermitic” and possibly even becoming a hero depending on how involved he gets.
Conversely he could help the invaders in exchange for being left alone after the conquest. The invaders probably don’t know the land that well, while the Hermit would be more knowledgeable about such things and could act as a scout to help make the conquest go easier. Depending on how helpful he was, the invaders might even offer him a permanent position in the army or something similar. Or of course he could just return to being a hermit.
Or if you really want be on your own, stay, but don’t take sides. Help or hinder each side as you see fit, this could be as simple as just killing anyone who comes near your bunker/cave/etc. or you could be actively going out and ambushing and robbing folks for supplies within the chaos of the occupation.
There’s a lot of things you can do with this idea, but those are some basics.
