I literally have just gone through this myself within the last month or so. Not telling was causing me a lot of anxiety. I guess I lucked out because he's got a couple mental health issues he was very open and honest about. So I just kind of said that I had something I needed to be honest about too.

I think you'll kind of know when the time is right. My opinion would be to tell him verbally though. If it's causing you anxiety to wait, then I would say something sooner rather than later. Any guy that likes and respects you will be ok with it.