Unfortunately, in the case of a cruciate rupture, stabilisation of the knee joint with a brace is not really possible, and does not really help.
The dog's leg is built in such a way that the knee is virtually impossible to keep still. A built-in mechanism means that every time the hock moves, the knee automatically moves too, and so even with a brace you cannot keep it still. There is also no real value in bracing the joint, because this will not make it any more stable in the long run. Dogs who tear their cruciate ligament often damage the cartilage lining of the joint at the same time, and it is often the damaged cartilage that causes the pain rather than the instability by itself.
As you are obviously aware, surgery is the best course of action for most dogs over about 20-25lb. In some situations you can get some improvemnt using antitinflammatory drugs such as the dermaxx but this requires strict rest of the dog and long term meds - and is not guaranteed to work.
You are absolutely right that your dog needs to lose weight - this will help the limb function, but just stopiing eating is not the right way to go about it. If your dog will not eat this is a sign that something is really bothering him and if it continues he needs to be examoned again or at least for you to call your vet for advice. I appreciate that you cannot afford expensive treatment for him, but if he is in so much pain that he will not eat, then he does need to be at least examined again.
I hope this helps,