Issue of land in family

We have an issue where the family has 5 kids. There is a gravel pit with land worth millions owned by the parents. The one son runs it. we were told 2 years ago that when the parents die the land would be split amongst the 4 kids and the son would inherit the gravel business itself. this past summer we found out that this was no longer the plan. the one son will get it all and if he sells the land someday he may split the money among the kids.
The son that runs it all recently took out a million dollar loan to buy another gravel pit and we think he had to use the land for collateral to get the loan and thats why this has happened. The son owns 49% of the gravel pit and the dad owns 51%. We all find this so unfair. the son is already rich so now he has taken what we were all promised. Is there anything that can be done upon the dad's death to fight this? what can we do? thanks so much