surgeons here in Aus have started operating with some ACLs in teenagers although the general concensus where i am located is to hold off as long as possible unless it's going to compromise the child's chance at elite/olympic level sports due to the skeleton not being fully fused. While surgical techniques are always improving there is still some risk of disruption to the epiphyseal plates of the growing leg and so you probably want to find a surgeon who has operated on skeletally immature teens before and see what their outcomes are like. An ACL reconstruction in the long term would be beneficial but check with a good surgeon about your son's long term development and growth and risk factors.