My boyfriends and his ex have a daughter together and she is now 6. the past 4 years we have tried to see her which only happened twice. As soon as the mother found out my boyfriend and I were having a baby she changed her number and lost all contact. We had sent money and everything to visit but when we would come she would leave with the child. As of three months ago the ex had taken the daughter to the grandfathers and left her and has not been seen nor heard from and the grandfather won't let my boyfriend have his daughter. What can we do legally?

HE needs to go to court and file for joint custody

What have you done "Legally" at this point? You don't "pay" for visitation. You go to court and get the court to issue a custody and visitation order. If you want full custody of the child, you go to court and file for it.

Legally, there should be a child visitation order in place, if mother refuses to allow, then mother would be taken back to court for contempt.

It sounds like he has been doing nothing though the courts??