His testimony can still be used in a grand jury indictment against Obama, which could be used to subpoena others who have first hand knowledge.
A criminal prosecution? Up until now, to the best of my knowledge, it has been all civil actions challenging Obama's constitutional qualifications, except the pending military trial of Lt. Col. Lakin.
If (hypothetically) a grand jury were to subpoena people in Hawaii state government who have knowledge of the matter, they could start with Governor Lingle and Dr. Fukimo, her appointee who heads the state Health Dept.