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The parent is obligated to pay support until the order is modified by the court. Moving back into the home and providing support to the child in that way can alleviate a parent from having to pay child support. The payor parent would have to petition the court for a modification, and will have to show that he/she provides support to the child by virtue of living with the child.
Yes, in that case, he was not supporting the child, and you can go after him for non-payment of support.
It is possible that he could get some credit towards his child support arrears because he was helping pay the mortgage, but that is not likely. In either case, he still owes the child support for that time period.
Regarding service of papers, you can ask the court to serve by publication in a newspaper. You have to swear that you made diligent search for him, and that you were unable to find him.