One parent shouldn't necessarily be "punished" for moving away. They have their own life, needs, job opportunities, love interests, etc. that impact their decision on where to live. It's unrealistic to expect the parent that doesn't have the child living with them to stay tied to one geographic area.
Making the parent that moved carry a bigger burden may seem fair, but it's actually as unfair as telling the parent that didn't move to also move. (Which also happens, when the primary custody parent moves hours away. The secondary custody parent often has to "deal" with it by putting up with increased driving, or moving themselves.)
It may be an unfortunate situation for you, because of increased driving time and expenses, but that's not how it should be looked at. This is really about the child and his ability to visit his father. If you have the financial and physical means to drive to the father's, then you should be willing to do that. You've been very fortunate thus far that the father has been closer, and you've been able to split the costs. That definitely makes it easier on your son. But that luck does not confer a right.
I think it's also important to keep in mind that the father's decision to move wasn't because he isn't dedicated to his son. These days, a 6 hour drive isn't really that far. His attitude of "demanding" is definitely poor, but you'll need to try to be objective about it.
You should be willing to split the drive, or even do most of the driving, if that's what's necessary for your son to maintain a healthy relationship with his biological father. Any set up that requires you to do less than 100% of the work should be considered a perk, not a right. Ideally, the father should also have this same mindset.