Living with Him is a slow-paced, slice-of-life, friends-to-strangers-to-roommates-to-lovers story. It's one of those very gentle J-BLs which are not so much about the story but more about the characters.

The strength of this drama lies in how it is showing the dynamics of two childhood friends who are most comfortable together when they just live in the moment, don't think about what they are to each other, don't think about how others perceive them. As long as they are in their own world, everything is fine. It's when one of them starts to overthink things as well as when they think about what they are to each other that this closeness falls apart, and suddenly there's this chasm that they feel can't be overcome.
But they want to be near each other, so they try to find that balance, only to lose it again in the next moment. And this is what makes the drama interesting, not the plot but the change of distance in an evolving relationship.

The actors are quite good at showing this; unfortunately the story telling is quite different. It doesn't manage to build up the kind of tension the premise deserves, everything feels a bit flat. The scenes where Natsukawa and Kazuhito are very close to each other have the same overall feeling as those where there should be uncomfortable distance.

What also does not help is that Natsukawa is not a consistent character -- he says he is glad to be free of his obligations to his younger sisters but proceeds to fill exactly the same role in his new household with Kazuhito. And the question remains: After he realizes his feelings, what is holding him back? This is more than just the usual Japanese communication difficulties. Does he think he's not worth it? As the one who took care of two sisters for years, and seems to be mature beyond his age, his sense of self-worth shouldn't be that low. Does he have some internalized homophobia? Then it should have had more room than just that small scene.
In my opinion, a focus on one solidly developed inner obstacle and a consistent characterization for Natsukawa would have helped a lot.

The drama is worth watching for the change in distance romantic feelings can bring to a life-long friendship, just be aware that this is definitely not fast-paced, and much of what happens seems very mundane.