I'm sorry you are going through this. Teenagers are difficult under the best of circumstances.

I have kids but no experience with divorce, so I'm not exactly an expert. My initial reaction is that keeping your youngest from being at his dad's is punishing him. Also, there are no guarantees that if the youngest isn't there the ex will spend more quality time with the oldest. It sounds like you all need family therapy to help navigate this and get our oldest through this time. Hopefully, he has learned his lesson and has been scared straight by the experience.