That you exist for your son to come to is already very crucial. I had a dad like that, but no mom. At his time of life, your son is building his autonomy and his dad sabotaged that work. That's SO destructive. The cold shoulder cut-off is SO typical, devastating and I just have always hated the way my dad always escalated any disagreement into Total War just like that. It just completely sucks. I know
exactly how it is.
Here would be my wish list in his shoes - maybe run it past him.
Your son needs support in building his individual strength as a decision maker and to build experience in feeling effective as a soon to be adult who will soon be running his own life's business and only seeking advice and input - i.e. not unilateral decisions and power plays from his 'rent.
I think you should be really honest in validating your son's point of view. It's really unfair of his dad to do his freeze out power play and I think it is fine for you to state that, but put up front that you are biased and involved. Facilitate him seeing a third party (therapist type) so that he can be free to explore the whole range of this stuff safely. Make sure he knows you and the T are not kibbitzing behind his back.
My husband's dad is a big fat controlling N and my husband turned out great anyway. His dad is his example of who Not to be. My H learned how bad it feels to be treated that way and runs his own life much more wisely and insightfully though of course not perfectly or else the stress of perfection would get to him.
Practical question for you: is dad/has dad said he would financially assist in college? Be prepared to have alternate strategies because clearly Dad is very capable of misusing his leverage.
Also, for college, your S should not worry that a bump in the road of his grades will put college out of reach. He just needs to make sure to interview for school and/or to also address the issue in his application essays - up front. It's easy to do great when things are going great, but it says a lot about a person's character when they soldier ahead when trouble happens like it is now.
hope these thoughts help.