Hey there, I am an Aussie, and while my kids are a bit younger, my current partner has had issues with his children who are the same age.
At around the age of 14, the courts would probably take the child's views into account, but they are very much guided by the Family Law Act, which is very keen to uphold relationships between parents and their kids.
Bottom line is, as loing as you as the parent are not actively encouraging him to stay away from his dad, and doing what you can to facilitate a good relationship, no one can force him to go. However, if your ex is wanting to push this, he will need to go to court. To protect yourself, I would keep a journal about everything from here on. I would also find ways for your son to find ways to express himself in a way that works for him, and perhaps find some counselling to work through these issues.
Your ex is not doing himself any favours by changing the schedule outside the court orders. I would be making a record of this too.
I think you are doing the right thing in having your lawyer handle this - taht way you cant be seen to be alienating your son.
i feel for your boy - it must be very, very hard.