I don't know anyone in that situation- but I do know that Grandparent's Rights are becoming more and more important legally in custody and divorce situations, as the situation becomes more and more common. My attorney told me that the laws are changing all the time, being added to and updated. Like most custody/visitation rights they are different in different states.
I suggest you speak to an attorney- it can be worked out so that you get visitation, particularly if you have been your grandchild's full-time caregiver in the past. If you cannot afford an attorney, go to a women's shelter, community center or your local clergyman and ask them for help and advice on who to talk to so you can make this happen. You might have to fight through some legal red tape, but you can probably get a hearing in the end.
You don't give a lot of details about the situation, so I hope and pray your grandchild is in a safe place with people who love him. If you suspect that he is NOT safe or that they won't be caring for him properly, it is all the more reason to establish that legally as soon as possible. Best of luck - my prayers and thoughts go out to you!