This is so hard feel for your son, I’m a dad who would do anything to have his son any extra and I get him 3 nights a week I just don’t get why dad’s do this.
Its a hard position for you because you don’t want to stop your child seeing his dad his dad’s just being an arse, he can’t just keep popping in and out of his life when he wants.
My advice would be to keep a little black book of when he does go with his dad how your child is when he returns then you may need legal advice.