I am 15 and just had my ACL reconstruction surgery done and my miniscus repaired. It would be wise to have the surgery because he has probably stopped growing at the age of 18. Thats a big factor in MCL ACL surgery because of your body still growing. My opinion.
In my experience, treatment of ACL tears is dependant on three factors: age, activity level, and degree of instability. In a young, active athlete, with an unstable knee, very few orthopedic surgeons would recommend observation and/or bracing for his knee.
Surgery is best performed approximately three weeks after injury, after the swelling and inflammation have resolved, and range of motion is restored. At the same time the torn lateral meniscus can be address either by repair or removal.
If your current doctor is reluctant to peform the surgery I dont think a second opinion would be a bad idea. As an aside, although many orthopedists perform this operation, it is always a good idea to inquire how many the treating surgeon does, Other issues to explore are what will the surgeon use to replace the ACL: hamstring vs. patella tendon, allograft vs. autograft.