Thanks for the information you have provided. A partial tear can heal on its own but the dog has to be kept inactive as possible and it still takes months and reinjury is highly likely especial with larger dogs. So if you wanted to take care of the tear, surgery should have been recommended . He is relatively young, so it is cost effective.
For arthritis, there are a lot of different options to treat. One is giving glucosamine. Many foods are now produced that have glucosamine to help with joing health.
You might want to consider Adequan which is injected and said to have some great results in restoring mobility of the joint.
And if the condition is extremely severe, surgery might be warranted. Surgery might include just clearing out debris from the joint, or repairing the joint by rebuilding parts and fusing bone or in some cases whole joints are replaced with artificial joints. Some owner elect to start using braces to help keep the joint imobile. You can see braces here:
There have been some interesting results in using stem cells in the treatment of canine arthritis. You can read about this here and talk to your vet about this possible treatment for your dog.
You might want to have your boy seen by an orthapedic surgeon to evaluate possible surgical solutions.
You might start feeding a very bland diet of boiled chicken and rice along with some nutrical to ensure he is getting necessary nutrients. Most dogs will eat boiled chicken (or well drained hamburger) and rice. That might help him get his weight back up while you ask your vet about alternative treatments for the arthritis and knee tear.
I hope this information is helpful to you. If you would like any additional information or have more questions please don’t hesitate to press the reply to expert or continue conversation button so I can address any issues you still have . If you do find this helpful, please take this opportunity to rate my answer positively so I am compensated for my time.