ANSWER: You tell him everything, you absolutely gush, you break down, you sob, you drag every ounce of everything you have in you to the surface, you mourn his death directly in front of him.
You leave no emotion unturned.
Very very few suicidal men want to die, but despair has taken hold of them. Sacrificial (Note that word CAREFULLY) despair. Where the only just outcome in their mind is their own end for the "greater good", for some benefit. They value their lifes as so expendable, so without value that the best they can do for the ones they love is end it so as to not be an impediment to future happiness, or perhaps go out in such a way that their loved ones financially or socially benefit.
Men are obsessive. They fixate. Don't even get mad at me, you know EXACTLY what I mean. And when they begin to fixate on how the most logical course of action in their life is their own end it spirals. Goes from being a fixation to a self fulfilling prophecy. They become a cult of one just psyching themselves into it, never out of it, waiting for the right moment.
A dreamy calmness comes over them. That's why he's laughing, planning for the future, having a good old time. He doesn't want your last moments with him to be sad, he doesn't want to tip you off or worry you. Chances are he wrote out the letter but kept it to himself so that when, after he lost his life through some accident that paid out well you'd find it, years later, and know the truth.
So the only force against such monstrous despair is Love. Pure Love. Brutal Love. The kind to make the Angels weep. Prostrate yourself and show him that the entirety of your future is at his mercy. To shock him out of that dream like haze of acceptance of his own end. To show him that the cost of his endless loss amounts for more than any pitiful benefit.
Good luck.