If it's just the ACL, he'll be fine next season. Especially with access to top-level surgeons, rehab, and training. It's only when there's collateral damage, particularly to cartilage, that things get dicey. Ligament repairs are very straightforward now, but meniscus and articular cartilage are still difficult to restore to original condition.
I've torn both of my ACLs, but was fortunate that I did very little damage to the cartilage each time. I was back to regularly programmed ski bumming in 6 months, without the benefit of professional trainers and primo drugs. It's a tough recovery process, but eventually you get back to normal.