I am a child of divorce, my old man walked out me when I was 6-he ran off with his girlfriend, started a new family with her, I didn't see him again till I was 10 and that was only because my grandmother wanted to see us again. She passed away shortly thereafter, that severed all ties between us. I can personally count on the fingers of one hand the divorced children I've known who had a proper relationship with their fathers. I think as far as he's concerned, you're an EX-son. I wouldn't bother with him.
I've have yet to know anyone who went from pro to anti-gun, rather I have known lots of liberals who were closet gun owners.
People change of course, usually not for the better. I have had several friendships just fade away, dry up and disappear. You cannot force a relationship with someone, and my philosophy these days is that if someone doesn't value my friendship theirs is worth even less to me.
I went through Special Forces training in 1969 after I got back from Vietnam, it was a bad experience, the training left a lot to be desired. It sounds like you received quality training from real professionals who took it seriously. Bully for you.