He is causing the problem if he never said this:
"Hi Dad, yeah, I'm not with hobbity-blobbity. Sure catch up with her if you like, but spare me the details and she doesnt need to call me to confirm anything... yeah that might cause problems with my current relationship... Naw, i wont elaborate, but just do that okay? Yeah, dad - have a great time!"
So, you (4s) shouldn't tell Daddy Dearest anything... Mr. SO is should be handling this.
It sounds like he is allowing something you've conveyed to him that you're uncomfortable with.
hth