Hi Jean, I would like to help you with your question.
It sounds like this situation has become a power struggle between you and your grandson's mother.
It is common for very involved grandparents to want to have more say so in their grandchildren's lives. After all, you did act as a parent to him all of these years and helped his mother with raising him. It is natural to feel you should have more input in his future as well. And considering you are paying for a lot of the education, it is nice to be able to direct how your money is spent.
But the main issue with feeling that way is that ultimately, your grandson is his mother's child and not yours. Your grandson's loyalty will always be with his parents first. That is natural and a very common reaction with children. Your grandson may see you almost as a second mom, but his mother comes first.
Because of your grandson's feelings about his mother, when he was put in the situation of having to choose between you and his mother, he chose his mother. Having to make that kind of choice most likely bothered him to the point he felts upset and angry. When his mother suggests you are at fault for the situation, your grandson naturally takes his anger out on you. He is too young to understand parenting and how hard it can be to invest yourself in a child and feel you want the best for them. He will get a clearer picture once he has children of his own.
In the meanwhile, you can fix this by letting him make the choice he wants to without telling him how you feel. Although you do know better, he has to make his own choices and mistakes. Let him. The important part of all this is not so much what school he goes to but how your relationship with him stays healthy and close. Long after he is out of college, he will still have you in his life. You want him to feel close to you and share his life with you. Back off, tell him you are sorry and that you only want the best for him. Then support him (and his mother) in the choice they make. Do your best to keep your opinion to yourself, unless you are asked. Even if you do not get along with your grandson's mother, you can still have a good relationship with him.
I hope this helps you,