I think you are going in the right direction. Treatment needs to be based on a diagnosis. Otherwise it is just as useless as blowing into the wind.
Symptomatic treatment based on history, physical exam and symptoms, which is what you did originally for the vomiting, works a lot of the time. When it doesn't work, other diagnostic tests are needed. This is what you are doing now.
The physical exam should be repeated including temperature, heart rate, respiratory rate, looking at membrane color and capillary refill time, listening to the heart and lungs, feeling lymph nodes and the abdomen. Blood work starting with a complete blood count, serum chemistry panel, thyroid and specific tests indicated by the history and physical exam and urinalysis give a good idea of major organ function. X'rays and ultrasound are imaging methods to look at internal organs.
Common diseases in older Boxers include ulcerative colitis (a type of imflammatory bowel disease), various types of cancer, cardiomyopathy, intervertebral disk disease, hypothyroidism, and Cushing's disease . Ulcerative colitis and cancer are the most likely to cause diarrhea and weight loss. Cardiomyopahty causes a precipitous weight loss plus signs of heart failure, but not diarrhea. The other common diseases are not generally associated with weight loss.
You should have some results from the blood and x'rays today. Let me know what is found and I will try to provide more info.