Ok, so you are the grandparent. Child support is for the child. If you have custody, you can get support. However, if you apply for custody, he may revoke the POA and take the child.

I don't think that would happen, since he gave you the POA in the first place. Its an indication that he doesn't want the child. And that he would just be asking for custody to avoid paying support. So I would recommend that you contact a Family law attorney to advise you as to whether you should file for custody and support.