Everyone else has pretty much said what I would -- be upfront, don't try to predict how he'll react, and know that either way honesty is your friend right now; it'll relieve a lot of the stress I know you've been feeling from hiding this.

It took me a really long time to tell my dad that I was depressed -- my mom knew, but I was afraid my dad would have a "get over it" reaction, because he's not really a believer in mental health so much as "mind over matter" -- and I actually ended up blurting it out at the airport about an hour before I was due to get on a plane across the Atlantic back to school. But he's been remarkably understanding and helpful about my depression. I don't think he completely understands it, but he tries his best. Sometimes we don't factor love into the equation when we try to predict other people's reactions. If someone cares for you, it's amazing how willing they'll be to support you, even with the things they find difficult to accept or understand.

Good luck!