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Hello and welcome to Just Answer. I am sorry Brodie is not feeling well. It is impossible without an exam to say what the problem is but you asked for suggestions of what it could be and there are several things that can be causing this but it seems that he does need vet intervention. If he eats any non food items such as bones, raw hides, toys etc. then he may have a blockage of the intestines, (you might also see vomiting and diarrhea).
If he is listless an infection may be the cause but in senior dogs tumors cannot be ruled out.
I would check his gums and if they are white or pale it is an emergency and the same goes for blue or gray gums .
If he is not eating then hypoglycemia may also cause listlessness. You should rub a little pancake syrup on the gums to help with this, but since it has gone on for a while your best bet is to get a physical exam by your vet as soon as possible and get the correct meds that may be needed. An infection in the body would need antibiotics.
Check his temp rectally for fever. Normal temp is 100.5 to 102.5 (38-39 C)