If you don't feel comfortable writing a letter, why don't you try bringing it up via text again? One of the days you know you are going to see him, just shoot him a text saying "hey, we still gotta talk about our relationship status! Maybe we could talk discuss it tonight?" or something along those lines. Then, when you meet up a couple hours later, he will be expecting to talk about it and hopefully you can ease into the conversation.

Alternatively, you could try talking over the phone if it's easier? Face to face is always the best way to communicate, but at this point I think any way you can get through to him is good!