Who knows if that would help, but perhaps it would also be useful to remind him that even if you're father is right, and you are truly wayward, that it doesn't mean he will lose you in the next life, as indicated in the following statement:
“The Prophet Joseph Smith declared—and he never taught a more comforting doctrine—that the eternal sealings of faithful parents and the divine promises made to them for valiant service in the Cause of Truth, would save not only themselves, but likewise their posterity. Though some of the sheep may wander, the eye of the Shepherd is upon them, and sooner or later they will feel the tentacles of Divine Providence reaching out after them and drawing them back to the fold. Either in this life or the life to come, they will return. They will have to pay their debt to justice; they will suffer for their sins; and may tread a thorny path; but if it leads them at last, like the penitent Prodigal, to a loving and forgiving father’s heart and home, the painful experience will not have been in vain. Pray for your careless and disobedient children; hold on to them with your faith. Hope on, trust on, till you see the salvation of God” (Orson F. Whitney, in Conference Report, Apr. 1929, 110).
If I left the church, I would want my parents to take this promise seriously, at least to hedge my bets. I would tell my dad that I am doing my best with what I know and feel like I can honestly believe, but that if I'm wrong, that I hope my connection with him will make it so we do not have be apart forever as a result.
Because, you know, you may be wrong. The safety net is kind of nice.
Cheers,