What are his Father's Rights?

Our son baby mom is threathing him of not letting him or us see his daughter. He provides for his daughter. He has never miss a child support payment. He buys whatever his daughter needs; toys, food, high chair, clothes, and he has set up an insurance policy, savings account, and plus his daughter gets tribal benefits. The baby lives with mom and they have never been married. Our son watches his daugther as much as the mom will let him. Plus, we watch her 3-4 days out of the week and then sometimes including the weekend. Our son has taken his daugther to the doctor I don't know how many times cause she is constantly sick.
Just last night, the mom got mad at us because we were unable to keep our grand daughter this weekend. The thing is we didn't even tell her we would even watch her this weekend. WeI think she must have had some plans for the weekend and we ruined them. We have only turned her down 1 time.
What would you do in this situation?

The mother is a big time partier. We have always been nice to her. We would love for the mom to give our son full custody but she doesn't want to cause it will take her monthly child support. She has never had a job and her income is from her two kids. Our son and us have done everything that she has asked us to do except for now.

First, you have not enough facts to give a relevant legal answer. So, answer first whether or not your son signed the Birth Certificate or Acknowlegement of Paternity.

If so, then he is the father and he has the same rights and responsibilities as the mother, i.e., he can file for custody and / or visitation and allow the court to proceed to award his rights.

At the present time he has what is called 'presumed' rights. He must follow through with court action for his rights to be legalized.

If he did not sign the birth certificate nor the AOP, then not only does he need to consult with an attorney, but he needs to stop giving this girl any money or supplies for the child. If he insists, tell him to open a bank account for the child's benefit then proceed with the attorney to file for a paternity test and attendent custody / support / visitation actions.

Without court action, an unmarried man has no legal rights to a child other than what the court awards.

Your son should hire an attorney, family practice. More and more men are getting full custody, and to a lesser degree, shared custody. With the right attorney, you can find out what his rights are regarding his daughter, and what he needs to do to pursue custody.

Every unmarried father in this country has the responsibility to learn what the law is, in his state as to the rights concerning his child. Many states automatically give custody to the mother, while the father MUST file in court to have his rights ruled on.
Unless it has already been determined by a court hearing, this is what he needs to do. The judge will determine if he has visitation rights, custody rights, support obligation (and how much).
You might also google to find a father's rights support group for your state. That is one place to find the exact laws, and perhaps how he can educate himself as to how to file a court petition successfully. What he already is doing, will count in his favor.