One subtle thing you could bring up with him if you decide to do so...
Lack of true love in the congregation. In a nutshell you can't seem to help notice that people (bros & sisters) just don't care anymore. You look around the Hall and can't help but wonder if Jehovah has removed his blessing..and its not just your hall but others too are noticing the same thing. Could He be rejecting us like he did the Nation of Isreal.
What would it take for him to reject us (JW's). This could open the door to finding out if your father has any pet issues that could be further explored.
It also provides a somewhat legitimate 'reason' as to your lack of activity.
Whatever you decide I wish you well!
Kismet