If you DS has ACL surgery and 12 months has lapsed since reconstruction and the knee is asymptomatic and stable they will qualify. Make sure you get good documentation. Most importantly will be the report that says "the knee is asymptomatic and stable" (I would also have added to the note released to full and unrestricted activity) .
If he only has an ACL injury than 6 months need to pass and I would suggest a physician's report that states "the knee is asymptomatic and they are released to full and unrestricted activity"
Disqualifying would be limited motion, and from
http://www.dtic.mil/whs/directives/corres/pdf/613003p.pdf page 33 Bold is mine.
d. Leg, Knee, Thigh, and Hip
(1) Current loose or foreign body in the knee joint (717.6).
(2) History of
uncorrected anterior (717.83) or posterior (717.84) cruciate ligament injury.
(3)
History of surgical reconstruction of knee ligaments (P81.4) (CPT 27427-27429) DOES meet the standard if 12 months has elapsed since reconstruction, and the knee is asymptomatic and stable.
(4) Recurrent ACL reconstruction (CPT 27427, 27407).
(5) Symptomatic medial (717.82) or lateral (717.42) meniscal injury. The following DOES meet the standard if asymptomatic and released to full and unrestricted activity:
(a) Meniscal repair (CPT 27403), more than 6 months after surgery.
(b) Partial meniscectomy (CPT 27332-27333) more than 3 months after surgery.
(6) Meniscal transplant (CPT 29868). (7) Symptomatic medial (844.1) and lateral (844.0) collateral ligament instability.