I usually don't suggest "ask an attorney" because we're supposed to give laypeople self-help advice, but your fact pattern is so abnormal and strange that there's much more going on here than meets the surface.
Usually grandparents don't get custody to begin with, against the wishes of the biological parents, and certainly without serving papers on them that makes them aware of a custody hearing and their right to participate. So I simply don't know what can be done, except discuss it with an attorney and be prepared to go into the strange circumstances here in some depth.
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Are you the father? If you are and you were not provided with notice of the petition and an opportunity to participate at the hearing, then you should file a petition for custody and get custody of the child. If the grandfather claims that you were served with notice but defaulted, then make a motion to set aside the default and ask for an opportunity to be heard. Non-parents can only get custody based on extraordinary circumstances. You haven't told us how he got custody. In any event, don't give in to the grandfather, what more can he actually do to you?
Because you do not identify the party for whom you are seeking advice, it is difficult to provide answer your question. "What can be done" would vary greatly depending on the party. The advice the father would get would be entirely different than the advice the grandfather would get. You may wish to consult with an experienced attorney (or you may want to suggest to the father or grandfather that they consult with an attorney).
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