Property law is the area of law that governs the various forms of ownership in real property (land as distinct from personal or movable possessions) and in personal property, within the common law legal system. In the civil law system, there is a division between movable and immovable property. Movable property roughly corresponds to personal property, while immovable property corresponds to real estate or real property, and the associated rights and obligations thereon.
Gotta use the 14th amendment for it and go after due process. Father has a right to counsel, right to be notified and such, in termination of paternity proceedings. Making plans to go away is not abandonment. If he was paying child support or sending money (and can prove it), then it is not abandonment. If the child was still under his insurance, it is not abandonment.
This may fall under the guidelines for kidnapping by deceit but you will not be able to get it into court. The government doesn't want to let that cat out of the bag.
This is a pretty common problem when leaving kids at a relatives house for long periods of time. I'm not a lawyer but I know enough to say he may have a long fight ahead of him.
P.S. It's not fair but it is the way the law is structured. If you ever have to leave your kids like that, make sure there is documentation or receipts or something to prove it is temporary and that you are providing for your child.