That's probably a bit much to start with, yeah. A letter or phone call, whichever is more comfortable for you, would probably be a better place to start.
Even email. The point is to get the ball rolling. (But keep a little distance right at the beginning, let him meet you halfway.)
He might be bi or he might have just been deep in the closet. It's not that uncommon for gay men to marry and really love their wives, even though they are in fact gay. (Becoming less common now that being gay is becoming more accepted, thankfully.)
But my point is more that saying that he "turned gay" is likely to get things adversarial right off the bat. Might be better to start a little more neutral and then as you get closer again, you can bring up things like that.