Your best bet is going to be what they call a TPLO. It isn't cheap, depending on where you are located likely $1800-$2500. There are some other procedures but you if you want a dog that goes back to work this is currently the most successful.

If your vet isn't sure you should get it checked bya specialist sooner, rather than later. The signs that show up on an Xray will be osteoarthritis because the leg is unstable. That is not reversible. Your goal is to get it repaired prior to that happening.