How about a sit down with dad- not to force him to pay, or force him to revisit the divorce decree. But just to tell him about your hopes/plans/interests for the future, ask him what HE envisioned your next step to be, and see where it goes?
If all of the communications have gone through your mom, he may never have had the opportunity to hear directly from you about your college aspirations. And maybe getting that dialogue going will be a productive next step?