You know his secret and you still love and support your dad. I would tell him, explain it slipped out and that he has all your love. He most likely will be over the moon, partly from your acceptance and partly from the relief of not having to hide! To be unburdened of shame and worry is a wonderful gift.
My daughter (17 years old) knows, she told me it has deepened our bond. While not every situation is the same, I can see nothing negative from letting your dad know. Good luck!
