Hillman last heard from his son in mid-July, when Sean left a strange message on his grandparents' answering machine. In a move unusual for the 5-year-old, Sean called his grandmother by her first name.
"He basically said he missed everyone in Seattle, but that he was going away and wouldn't be able to see anyone for a while," Hillman said. "I knew he was being coached."
Days later, Hillman received a threatening e-mail from a Japanese attorney retained by Ogawa, claiming he'd face stalking charges if he attempted to contact his son.
Fearing he'd lost Sean, Hillman phoned the attorneys who had represented him during his divorce, Michael Louden and Amy Franklin-Bihary. They and lawyers in Japan began fighting Ogawa's legal action, ultimately garnering two favorable rulings in Japanese court.
At the same time, Hillman continued trying without success to reach his son. Ogawa, he said, rebuffed efforts by State Department officials to simply see the child and refused to allow them to deliver Christmas presents to the boy.
Hillman has also reached out to Washington state's congressional delegation, prompting Rep. Jay Inslee to write the State Department in support of Hillman's claim.
"I described the actions of the Japanese government as predictable because I have had other constituents present similar cases and I believe Mr. Hillman is correct," Inslee said in the letter. "No similar case," he added, "has ever resulted in the return of a child to a U.S. citizen parent."
Louden said he and Franklin-Bihary have done nearly all they can from the United States.
"The only way that I think Sean is ever going to have contact with Carl again is to have the Japanese government take action," Louden said.
Japanese consular officials did not return requests for comment on Friday because of their homeland's vernal equinox holiday.
In a statement to ABC News responding to a request last year, the Japanese embassy staff offered their government's sympathy to parents but defended the country's position on the Hague Convention.