Let me put it this way son. If you don't have the time to make the trip to ask him face to face for his daughter's hand in marriage, then he's probably not going to think you have time for his daughter.
There are no do-overs in this situation. Marriage is a huge commitment. Show him you're the man for such a commitment. Tell him you need to come up and talk to him and you don't want your girlfriend to know about it. He'll figure it out, sure, but that doesn't lessen the occassion, and he will only appreciate it more.