How to get granddaughter's father to pay part of her College tuition?

Her father is divorced recently from the mother and he promised in writing that he would pay the tuition for either a city or state college (NY) They reside in NYC. He earns over !25,000 per year and never helps his children with clothing or anything else. We the grandparents have been helping although we do not have a lot of money. He pretends he doesn't notice that his children need clothing or anything. The mother suffers from manic-depressive illness for years...She is unemployable even though she wishes she could get a job. Both parents are in their mid forties. Grandchild was admitted to UVM a state college in Vermont, and according to the divorce decree he is supposed to pay up to the amount he would have had to pay had daughter gone to a city or state college in NYS . He seems to have forgotten or pretends to have forgotten that he promised this in the divorce decree. She is starting college in the end of this August and he refuses now to pay for anything. How can we get him to help with this very soon because daughter will not be able to register for cles. He told her the other day that she should take out loans and that she would be very PROUD of herself because of this. He is a very greedy man and doesn't pay for anything if he can get away with it. Wonder how we can get this resolved in a hurry..He only told her that he wasn't paying just the other day. My granddaughter as well as her two brothersre always afraid to ask for anything from him. He has never encouraged them to go to college and the two boys so far have not even attempted to go. He is intimidating and yells and carries on, so they avoid him. Granddaughter is an excellent student and doesn't get mixed up with any kind of trouble. She has applied for student aid and has received some, but she was so sure that she was going to receive at least some financial help from the father. Help...Thanks.