Hello, my name is***** and I have over 20 years of experience as a veterinarian. I am sorry to hear that your fellow is having such a rough time of it the past couple weeks, with lameness and changes to his shoulder joint radiographically and now suddenly pale gums, weakness, lack of an appetite, possibly swollen abdomen,and lethargy.
I am not sure that his previous lameness is related to what you are seeing today. I would have to guess that his previous lameness was related to bicipital tendon inflammation/bursitis given the radiographic changes and that it responded so nicely to nonsteroidal anti-inflammatories.
Unfortunately Goldens have a higher incidence of cancer then many other breeds, specifically hemangiosarcoma of the spleen and lymphosarcoma that can invade the abdominal organs and lymph nodes. These tumors can press on the stomach and intestines and make them feel full or, especially the hemangiosarcoma, bleed with increases in blood pressure which would make him feel lethargic and explain his bloated looking abdomen and pale gums. I know that he was seen recently, but it possible that he had a tumor that was too small to palpate at that point given his big size in general. A growing mass would also explain his recent weight gain. The problem with these tumors is that unless they start to bleed, or get big enough to press on other organs or the intestinal tract they seem to feel fine, so we don't know to look closely for anything. We often don't find them until they bleed.
I think an examination, some recheck blood tests, some radiographs of his abdomen, and an ultrasound of his abdomen are great places to start. It will allow them to look at organ structure and look node or internal organ changes and collect aspirates of any abnormal areas. Radiographs are pretty good at looking at organ size and placement, and seeing big changes, whereas an ultrasound will allow visualization of more subtle changes. Ideally given his pale gums and lethargy today he would be seen on an emergency basis, but I realize that may not be possible. But I do think that you need to take him in tomorrow to be rechecked.
I would recommend that you keep him very, very quiet tonight.
To try and settle his stomach today and improve his appetite you can give either:
1) Pepcid-ac (famotidine) at a dose of one 10mg tablet per 20 to 40 pounds of body weight every 12 hours.
2) Prilosec (omeprazole) at a dose of one 20mg tablet per 40 to 80 pounds of body weight every 24 hours.
These will reduce stomach acid and can help settle his stomach. They are quite safe and can be used few days.
Offer a homemade bland diet to stimulate his appetite of 1/3 boiled, lean hamburger or boiled, white, skinless chicken, all fats drained off the meat, and 2/3 boiled, white rice.
Please let me know if you have any further questions.