Azathioprine and Metronidazole both made my boy feel very bad. With the Azathioprine I gave him one dose at bedtime and he was restless and clearly nauseous all night and didn't eat again until the following evening, so I didn't give him any more and we managed his autoimmune issues just on prednisolone. With the Metronidazole I suspected this was affecting my boy's appetite (I have taken this myself and know how sick it made me feel) and his appetite did improve once this was stopped.
Our situations are not identical, but I was in a similar situation to you a couple of years ago with the dog mentioned above who had returned from two weeks in hospital looking like a walking skeleton, with little appetite and bloody, liquid diarrhoea. His appetite was so poor that he would only eat a tiny bit of any one thing at a time, so what I found worked best for him was to offer him ten course meals. I kept a list of any food he showed any interest in (about 20 items) and at each meal (5 times a day) I would start at the top of the list and keep offering him different things until he'd eaten enough. I'd start with the healthiest things and let him eat as much as he wanted of those first, so a typical meal might have been: a mouthful of kibble, a spoonful of wet dog food, some chicken breast, a few mini cocktail sausages, a squirt of cream cheese, a tripe stick, a strawberry probiotic yoghurt drink etc etc. I avoided anything too fatty, as prednisolone does increase the risk of pancreatitis, but apart from that I gave him anything he'd eat. I was also giving him a probiotic paste (Pro-Kolin) and a concentrated vitamin/mineral paste (Nutrigel). It was a long slog, but over the course of a few weeks his appetite gradually returned to normal. We never really knew what caused this state of affairs, but I suspect it was the huge number of drugs they gave him whilst he was in hospital.