Mimi, i think i can sort of understand what you are feeling, sort of, since i don't have a grandson, but i have a son, and that kind of love is so big that is almost hurtful by itself.
I don't know if this helps, but i'll try.
I'm my self a mother of a boy, and i separated from his father when he was 3 years old. Altougth we never lived in the same citty of my ex bf family, but pretty close, and we visit them all the time, and they visit us frecuently.
I have move around my country because of my work, and i know my son's family sometimes miss him - not saying as much as you, since you raise him.
One thing is being dificult to me ( i would be like your daughter-in-law in this case ) is that we had a dificult break up, you know that when you finish a relation it's a problem of the two involved, and there are many bad stuff that happened inside that are dificult to forget, and to forgive.
Wheter we want it or not, we kind of make the family to share those guilties, and we translade to them some of our bad feelings.
So, first of all you should have talked to her, before (i'm not sure it's time yet but you can give it a try), about the relation with your grandson - wich is completely independent - on the relation with his father , or should be, you know what i mean?
Maybe they had diferences that canot be solved, or she and your son has issues that they have to resolve by they self, you try to ask her to see your grandson YOU, by yourself, because you want to give him the love he deserves.
Don't ask your self if she is rigth or wrong when it comes about your son, because you cannot be a fair judge. So leave it to them.
After that, i believe she can react better that she has done as far. Offer to her your help, but be honest when you do it, you have to mean it because you know we - woman- know when someone is being honest with you.
Remember that this is not about you, your daughter-in-law or your son, this is about your grandson, and you have to do what is best for him, and i believe (unless you see that his mother is making him some damage), that for a child the best is to be with his mother.
And that you have to support the only one person who will be there for him all of his life - and you know it, since you are a mother.
I'm not saying to take away your backup from your son, but try to separate those loves you have. I believe that you have to let your son to figth his own battles, and to find the understanding with his wife by him self. You can be backing him up, giving him ideas, but don't be in the middle, it's not good even for your son, he is mature enought to take this problem in his hands.
I don't believe this would be about lawyers, those law figths migth cause a big energy drain for all of you, and be hurtfull for your grandson.
Maybe you can get a good setlemen, talking with her in your best terms, and always appealing to the well being of the boy.
I wish you the best!
- Just to let you know, my son is now 11, and he spend almost all of his vacation with his father and grandmother and all the family, despite, he has been having economical problems for almost 5 or 6 years, and i have been the principal support for him.