Hello mjk73: I'm sorry you are confronting this most difficult situation. Presumably, by now, you've had your lunch meeting with your son. Hopefully it went well. I don't know as I have any particularly useful suggestions for you here. From my perspective, although in many ways your son is still a child, he is old enough that you really can't force anything on him. And trying to do so is simply going to make everything just that much worse.

So my opinion (for what it's worth) would be that as long as your son's dad is willing to have him, & that's where your son wants to be, then it's incumbent upon you to let it happen... even facilitate it to whatever extent is needed. Yes, things may not happen at your son's dad's home the way they would have at yours. But realistically this is simply the way it is. It's not something you can control.

The reality of the situation is, in a couple of years, your son will be a legal adult. And then you will have no control whatsoever. And if this all simply feels too overwhelming for you to come to terms with, then seek the services of a counselor or therapist with whom you can process the changes that are occurring in your life. I wish you, & your son, well...