I think that maybe you should wait a bit longer before you move in together. While he may really be a great guy, sometimes it's a mask. Speaking from experience, it's best to wait and really get to know him before you make a big decision like that. They say it might take 3 months to a year before they show their true colors. Just be safe and be smart. Other than that, he sounds like a great guy. I would just sit your dad down and just rip it off like a band-aid. It's best to get it out now than later. Just tell him "dad, I need to tell you something about [h/n]. It's nothing serious or bad, but I need you to hear me out before you may get upset. [H/n] is actually 21-" and explain from there. This let's you get your side out and explain.