Here's a possibility: talk to your LBS, where you think he'll be getting the bike. Personally, I'm a big Specialized fan, so I say go S. Anyway, talk to the shop, tell them what the deal is and see if they'll let you bring a bike home and then return it so your dad can go ride test ride bikes. That way, he's got something to open/unwrap on his b-day, but you're not committed to something he's never tried out.

I think it's great what you and your mom are doing!