He sounds unstable or at least very threatened by something.
Does the girl not drive?
OP, you said that the father isn't allowing them to get married until your son graduates. I think that's absurd.
They're adults, not teenagers. What makes him think that his daughter and your son will still want to marry when he is done with medical school?
That's a long time away and a lot of things can happen... Maybe he really doesn't want them to get married?
Odd story.
Does the girl not drive? OP, you said that the father isn't allowing them to get married until your son graduates. I think that's absurd.
They're adults, not teenagers. What makes him think that his daughter and your son will still want to marry when he is done with medical school?
That's a long time away and a lot of things can happen... Maybe he really doesn't want them to get married?
Odd story.
. DS was 21 when my first grandson was born. Initially I thought it was the end of my world, then I adjusted, DGS10 was born and I was looking forward to enjoying my new "grandmother" status with the kids and their new baby. 



). That's not even mentioning the fact that she's 21 years old and isn't his property to be locked inside his house away from you regardless of who is footing the bills.
Maybe the Dad had a disagreement about that as well- leading to his overreaction?
