Jaye - as Charis said already - it may be stress related - at least partially. If you have a nervous bird from character, weight will always be an issue. Our Willy f.e., one of our PMV birds had lost an awful lot of weight when I finally could hospitalize him and was near to death. He gained weight only very slowly and then only 5 gr-wise or less. Each time when he started to vomit he lost again what he had gained and I had to re-start again to feed him up. I did it with baby cereal with fruit to create a slightly acidic environment. Sometimes I added chamomille tea, sometimes cinnamon tea. Each time it worked perfectly. I started with 5-6 meals (every 2 hours or so) with 10 ml and then went to 3 times 20 ml. I did this for 4-5 days and then started to replace the lunch with de-frosted peas, 20 pcs. first and slowly increasing the quantity.
Willy has totally recovered but will never be the same weight as f.e. our Jimi. We let Willy run outside for the first time yesterday and he was so excited that he got diarrhea. But I knew it was only his "nerves" just like with a human. Today he was absolutely normal again.
So I think all those medications put additional stress on your bird's immune system so that weight will be an issue for some time. I think giving him something to eat which is easily digested would help perhaps. And don't worry if the poops are getting lose - this will firm up with different food again.
All the best for your little patient