I have been through this more times than I can count with my personal pups as well as foster pups. Yes, it is diagnosed on exam as Tayler said above but they will do an xray at the specialist to see how the knee is aligned for future surgery. When they fix the ligament, they align the knee to prevent this from happening again in that knee.
You absolutely should take him to an orthopedic surgeon for an exam and, if surgery is needed, I highly recommend it because otherwise he will end up with horrible arthritis. If you don't do the surgery he will eventually walk on it but unlike people he will have ongoing problems. Physical therapy for torn CCL (ACL in humans) is ok for some humans but it is not for dogs.
If you need help finding a surgeon I know one who has a clinic in Maryland if that would be doable. He is the best and has done my pups knees since 2006! Or, he might know someone close to you.
This link is an article written by him. It is an old link and he no longer owns that hospital that is linked in it....but the info is valid....
Yorkie Haven Rescue - Cranial Cruciate Ligament Tear Info