Crazy? Noooo, well if you are, rest assured that I am crazy too!
My BF suffers from severe depression and is one of those men who retreat to his mancave. When its very bad he has retreated for up to a month!
He also tried convincing me he's not worth the effort, but he definitly is, and then some. Lol
He was just used to women not respecting his need for space and eventually walking out because they weren't willing to deal with that. So guess he thought I would follow the same path...
Its not easy, and no, friends rarely understand. People have become sooo accustomed with regularly contact that anything less than texting maniacally on a daily basis is often seen as a slight to your partner.
In the past relationships fared quite well on daily phone calls, weekly letters, etc... so idk what all the panic is about a GF or BF who doesn't make daily contact multiple times.
My bf? He goes quiiiiiet, shuts the whole world out. Me? I send him a "hey thinking of u" text, or kissy face emojis, or find inspirational, funny, cute pics online, and send them every second day or so.
This way he knows I support him, but there's no pressure from my side for him to respond.
This is admittedly not the easiest thing for a partner to adjust to, but he's supportive of my MH needs, so its only fair I do my best to be supportive of his. And sometimes he reallllly needs to be alone.
Like your bf, mine also texts a short reply from time to time. Its his way of showing appreciation and making an effort to stay present. But by now I know him well enough to know he's not starting a convo, so my reply to him is usually a kissyface emoji and then that's the end of that

In other words, I just acknowledge his message.
Ps. I wasn't always this ok with his need for space. But after watching him do nothing but silently lie there face-down ignoring his phone and the door for 16hrs straight... I felt like an a.s.s for thinking it was just me he needed to get away from. I'm no stranger to depression, but seeing him look like shyt first hand put things in perspective for me real fast.