the last line you write says it plainly............."he's always been a good friend, would do any thing for you type of guy"............what's the problem?
you're ALL lucky to have him as a friend. get over it!!!!! he's not doing/trying anything sexual with you; he's having consensual sex with another adult. so?
I don't really get it, If the guy had mansex, big f-n deal, but it's all conjecture, the guy "who knows they Fucked" obviously has been down that road before, and good friends are not a dime a dozen.... If in fact you are a friend , talk to your buddy, it will be better if he can be open about who he is, eliminate that awkwardness, and if he's not hitting on you you may be able to get some "queer eye" advice.
Whats his friendship worth to you? are you worried that in this age of whats right and whats not that you will be subject to ridicule for being his friend? A little over a year ago I was caught in the act with another guy, by one of my best friends, he said it wasn't his thing, but he didn't care, and wouldn't say anything. But it wasn't long untill he and the others started putting distance between me and them, and after about 6 months they stopped coming around or even calling.
Don't be a jerk to him. Keep what you know to yourself, and keep being his friend.
Belive me I know what it like after my secret was found out, friends didn't have anything to do with me afterwards. No phone calls, no stopping by, snide remarks when walking past a group that they were in. Don't turn him out, good friends are hard to find, and the thing of it is, he works for a living, he has to eat and sleep just like you do, only the sexual thing is different.