We received a DS-2019 letter which contained all his information from the original DS-2019 form from the official program coordinator, which states that he is subject to the 2-year residency requirement. That's how we know.
He recently stayed in the residency country for over a year, so we have passport stamps/plane tickets from that time period. For older years, (he's from Kyrgyzstan), we have documents with his official address registration in Kyrgyzstan (internal state ID card, registration forms, and his military ID card). Our lawyer is also preparing affidavits.