Use the last known address to serve him. File an enforcement proceeding seeking child support, finding of willfulness, incarceration, money judgment, legal fees and interest. If you want an upward modification, then file a petition for it.
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If he is in the military contact his commanding officer. If, however, he is employed overseas and not in the military, then you would be wise to consult an attorney as different countries have different rules for service of process and some don't recognize US courts at all.
As long as the "child Support agreement" was reduced to a court order, then there's no need to re-establish jurisdiction. You would need to petition for upward modification after the 2 years, and then hire a private investigator to locate him.
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