Hi SonOfaSci.... I was never in, and don't know any Scientologists, so I'm not able to advise with expertise, but I'd like to know if you are able to spend any quality time with your Dad...or is he kept too busy by the CoS? I'm just thinking that, if you make the most of your time together, he may value your relationship enough to resist the pull deeper into the cult. .? Just a simplistic thought, I guess.
Hope all goes well.
ADDENDUM: What I'm saying is, even if getting him out is unrealiatic, just try to get as much time relating with him as you can within the constraints. If he's sure you're not trying to get him out, he might feel confident enough to maintain a good relationship.