It’s not uncommon for people who are trying to live together for the first time to have issues like these. You all did a great job at the outset. Setting up clear guidelines and penalities is usually a way to prevent situations like this. You didn’t mention what the penalties are supposed to be but it certainly sounds like it’s way past time to invoke them.
This guy is in control of the entire household. He does what he pleases. He has you all living on his terms. When any of you call him on it, he turns it back around on you by calling you names. He’s even dictating how often you will all see a counselor. Who made him king?
I’m concerned that somehow your other roommates have put you in the position of the “enforcer.” Your problem at this point is as much with your girlfriend and your other housemate as it is with Mr. Slob. It’s not just your problem. It’s a household problem. The three of you need to hold a meeting and decide what you want to do collectively. If you need help with that, by all means go to see the school counselor by yourselves and come to agreement. You don’t need him to go along to the counseling session. In fact, my guess is that he’s not really interested in changing but only in proving to the counselor that you are all being unreasonable. You’ll probably get more done if you leave him out, at least for the first meeting.
You might also see what your legal rights are since he did sign a contract with all of you that he is happily breaking.
Once you’ve decided on a course of action, the three of you need to present it as a group decision with group consequences. Be sure that everyone truly buys in and will back you up. If they won’t, then that’s the issue you need to talk to the counselor about. It’s not appropriate or fair of your housemates to put you out front, leading the charge and then to go passive on you.
By the way: Calling his mother might be one alternative but, frankly, I think that’s a last resort. See if you can handle it as the adults you are.
I wish you well.