People saying you cannot walk on a torn ACL, you cannot be further from the truth.
I tore my ACL (badly), and after a few minutes I was able to walk on it fine and drive 2 hours back to St. Louis that night. The ACL isn't responsible for helping you walk. Now if you told me he was skipping off the field sideways then I'd say that's a good sign he's ok.
My knee was diagnosed as NOT torn on initial inspection. Surgeons and trainers can usually tell that the ligament is gone by just twisting your lower leg side to side and pulling it out. Most cases this is true, but I can attest it is not always conclusive.
I was not concerned last night. I'm very concerned now.