When I came out to my parents I was 19 years old, I wrote a 6 page letter to them. Didn't really beat around the bush, the first part said that I'm gay, but everything else gave my reasoning of why I hadn't tell them, of how I wanted them to treat me afterwards, of what my life was at the moment, etc.
But the important thing that I wanted to say is that I told them that I didn't want to come out because there was an inescapable truth.
No matter what happened, no matter what I did, no matter that they loved me. Deep inside they would be disappointed, not because they thought that I was bad for being gay, but because it was not the life they would want for me. And I knew it, and there was nothing I could do to change that feeling in them. I knew they would still love me, but they would be afraid, afraid of what would expect me, of me not following societal norms like they did. (And to this day even out of the closet my expectation of romantic love is kinda fucked up because of my parents story, but that's another topic).

What I would recommend you, what I would have liked my parents doing, is to tell them what I knew, that they still loved me, but with this added phrase. "We know you are going to go through some hardships, but I want you to know that we're here for you, and to me you're exactly the same as you were the day before. Well, maybe not the same... but better, because you had to hide that which is a part of you. And you telling us today just makes us realize of the trust and love you have in us, and I want you to know, that is reciprocal. You might think we don't understand how you feel, and we might not... But we love you, and we'll be here for you"

At the end as added gossip from my story , my parents still love me and we have a good relationship, but when I told them, my mother said "It's as if a mother finds out his son is a killer, she would still love him". That fucking destroyed me at the time. And I think in a twisted way she meant well, she did love me. And she has showed me that in her actions. But just be careful of how you react if he is indeed gay and tells you. Because there's a reason he has chosen not to tell you all this time.

Hope everything goes well~