My husband experiencing pain behind knee.and humstring tendons feels to tender.what can it be and what to do to treat this condition?

Strain/sprain: It is likely a strain or sprain with possible tendon tear. Inadequate stretching before strenuous exercise or some type of fall may have caused some damage. If there isn't significant swelling and bruising, he should follow the r.I.C.E protocol. Rest, ice, compression and elevation. Ice 10 minutes several times a day and can wrap with ace bandage.