If you expect your S to honor his agreement with you then it sit down with him and come up with a payment plan. He should be respecting the commitment he made about that. However, don’t bring up anything about loving the GF more than you. His relationship with her is not a competition with you. It’s odd that you would even try to equate the two.
Finally, we don’t expect our children to pay us back for college costs, but if I had that agreement with them I would expect it to be honored.