Real estate question

My grandparents and great uncle owned fifty percent each of a piece of property up in Maine. When my great uncle passed away, my grandparents got a loan to buy out my great uncles half. They talked my father into putting his name on the loan, promising him that if he took over the loans, they would give him the entire property, but for now, their name would stay on the property title. So, my father took over the loan and is still making the payments. However, my grandparents changed their mind and decided they were going to split the property among all the kids, and also claimed that due to property value increases, my father's loan wasn't 100 percent of the property anyways. He has put so much money into this property, and now is getting almost nothing. Is there something he can do?