Unfortunately no one can guarantee how he will react so it's hard to suggest something. However, I would suggest speaking with confidence and from the heart as it makes what you are saying easier for the person to connect with because people are naturally drawn to authenticity. The ideal conditions would probably be a casual environment where you two are enjoying a conversation, and you bring it up by saying "hey, there's something that means a lot to me that I want to share with you" and you say the rest from the heart.
Basically what the above poster said. It would probably be good to just bring it up while you're talking to him and just be honest. You could say "This shouldn't change anything, but I trust you and I just wanted to let you know I'm gay." It's easy to get nervous when doing something like that though so if you do try to tell him and you miss some opportunities because you get nervous, don't get too worked up about it. That's normal. Just try to be brave and if he's a good friend and you trust him he should be fine with it. If you don't feel comfortable doing it face-to-face though you could do it over text (although face-to-face is really the best so it doesn't just become an unspoken thing as soon as you're done texting him). A letter or something could be another alternative. Hope this helps and good luck!
