That's too bad.
If writing personal things like that on Hive would be too uncomfortable for him, another option you could suggest after he has recovered from the loss is some kind of private online journal. I use Day One, which is on my Mac, my iPhone, and my iPad and it syncs with the cloud. It is encrypted if privacy is a concern. That gives a little more redundancy than a handwritten journal or one only on a single PC. I'm sure there are similar programs for the Windows or Linux world.
Here is Day One if he's an Apple user and you'd like to suggest it to him. → https://dayoneapp.com/
A big part of it for him was the "manual" handwritten side of it. As a non-writer, it was his way to connect with it and make it personal.
Yeah, I totally get that. I do the same actually. I think I mentioned on a different comment to you that I use fountain pens and enjoy writing longhand—and I keep a journal for my kids. One hopefully they can read someday if they learn how to read cursive (the oldest can already read cursive, so he's good, unless he forgets). So I completely understand both his desire and his heartbreak at losing it.
I'm just thinking if he wants to try something else, Day One is a cool option.