If the two of you have been friends for 50 years, I don’t think he’s going to end your friendship if he finds out you’re gay. Maybe he already knows, but hasn’t said anything because he’s respecting your privacy. On the other hand, if he doesn’t know, there’ll be a bumpy period of adjustment: not so much because you’re gay (unless he’s a homophobe to begin with), but that he’s missed the fact for half a century. No one likes to feel stupid, or worse, deceived.
Telling him you love him is a whole other issue. Best friends... of course there’s love there, whether he’s ever consciously thought about it in those terms or not. He loves you as a friend, as a brother, possibly even as family. But that’s not the same as a lover, or spouse.
There’s no way for me to offer good advice without knowing your friend and his circumstances: is he married? Widower? Has he talked about missing sex and affection? For that matter, I don’t know enough about your mutual history: have you been casually naked together, or jerked off together before? All of these are factors that would influence how you should or should not proceed.