They Id say go for it. At least try writing it out, see if it feels right. Can always shitcan it and tell him in person if it doesn't come together right.
The one thing I'd think about though, in saving awkwardness, is if you got the letter from him, what would follow next? While the initial reveal would be at a distance, would the next meeting with him be less awkward or more/ etc.? Judging others by ourselves doesn't always work, but it gives us a starting point, anyway.
I've never done it that way, either, so no personal experience on that front... Aside from 'never write anything down, you don't mind everyone knowing'. ;)
Also from experience... Guys tend to get all weird about rape. Seriously weird about it. So I can definitely understand wanting some distance in between / let him thrash it out on his own & not have to deal with that particular domino effect. Personally I go for the opposite. Have found that what works best for me is talking about anything dark -with fangs or without- in bed. Not in the act, clearly, unless unavoidable :p But Lots of skin to skin contact / gives them something to do (& Im okay with all 3 common reactions; sex, fierce holding, their getting up to storm about &/or wander = gives me the opportunity to take a shower or make sammiches in natural transition... Keep things casual on my end while they spin out & the suggest we hit up the range/ gym/ break/ bar/ whatever seems like a good outlet) / etc.
