Very likely the offer is contingent upon the successful completion of a background check. A thorough background check (criminal, education, credit) can cost a few hundred dollars, so firms won't proceed until an offer has been accepted.

Even though he was exonerated, his prison stay and other convictions may show up. He can explain, but a firm can rescind an offer for whatever reason they want (outside of protected statuses, which does not include criminal background). For a security position firms may want perfectly clean backgrounds.