Hi There! Sorry to hear about your Harley.
I had a poodle have this surgery at just under a year and she bounced back very quick... they said the young ones usually do... She was on rest for a few weeks and then did physical therapy which is very important, best if you can find a place with water therapy... and then she got back to normal very quickly. She is almost 5 now and has had no issues since... you would never know she had this surgery, she runs, jumps, and plays normal.
I would recommend getting your baby on a good supplement as soon as possible (Cosequin or Dasuquin) to protect this knee after surgery and his other knee as well.
I think it was about 2000 dollars for everything (xrays, surgery, physical therapy, post op appointments)
I am very sorry you are dealing with this but your Harley can make it through this and still live a very normal life
Please let me know if you have any specific questions, I would be happy to answer if I can.