They can hold him up to 8 years. That's the contract he signed....
10. MILITARY SERVICE OBLIGATION, SERVICE ON ACTIVE
DUTY AND STOP-LOSS FOR ALL MEMBERS OF THE ACTIVE
AND RESERVE COMPONENTS, INCLUDING THE NATIONAL
GUARD.
a. FOR ALL ENLISTEES: If this is my initial enlistment, I must
serve a total of eight (8) years, unless I am sooner discharged or
otherwise extended by the appropriate authority. This eight year
service requirement is called the Military Service Obligation. Any
part of that service not served on active duty must be served in the
Reserve Component of the service in which I have enlisted. If this is
a reenlistment, I must serve the number of years specified in this
agreement, unless I am sooner discharged or otherwise extended
by the appropriate authority. Some laws that affect when I may be
ordered to serve on active duty, the length of my service on active
duty, and the length of my service in the Reserve Component, even
beyond the eight years of my Military Service Obligation, are
discussed in the following paragraphs.