Very sorry to hear about your situation. Does your son agree with you that a change needs to be made regarding his girlfriend's treatment of you and the baby or is he siding with her? If he's siding with her you can file for visitation rights in Ohio based on the fact that it would be in the best interest of the child. However, it is difficult for grandparents to overrule the wishes of the parent especially when your own son is alive as he's supposed to, in theory, cultivate the child's relationship with you.
If your son is in agreement with you, he should file for a reallocation of parental rights and responsibilities. You can then join him in his case. The more parenting time he gets the more time the child can spend with you.
Generally when your son has visitation with his child he can bring the child wherever he wants within reason (ie: not out of state). If he wants to bring the child to see you that's up to him. The problem is that absent a court order, his girlfriend can simply stop visitation. You and your son should both talk to a good family law attorney and file a motion in your local juvenile court. I wish you luck.
Also, if you believe your grandson is in any immediate danger call childrens services and file a report stating what you believe is the problem. If the danger is severe, call the police and file a report.