Thank you for the additional information.
In a senior dog with these symptoms it is very very important that we get blood work to look at organ function mainly the liver, kidneys, and pancreas.
Pancreatitis is a real possibility and would explain all of the symptoms you are seeing including the painful abdomen.Left untreated this will effect every organ and can be fatal Click here
Considering the severe weight loss it is also important to get x rays to look for any abnormal growths in the abdomen signaling cancer or changes in the organs.I would also want to get a look at the spleen as large breed dogs are prone to tumors on the spleen.
Things like tick borne illnesses can cause this type of weakness and problems with the joints.
It is possible that the leg is unrelated to the other symptoms and is due to a problem in the cruciate ligament of the knee due to the stress of breeding.
At this point there has been no tests done and to be honest it is somewhat disturbing that this was chalked up to breeding weakness at this age. Without at least blood work it would be impossible to say what is going on.
I would suggest getting back in to the vet or getting a second opinion and getting blood work and possible x rays done.
I know you were hoping for a more accurate answer but I really think further immediate vet care is needed.
Please let me know how he is doing. Please reply back if you have additional questions or concerns and I will be happy to continue.