For sure the vet that is standing in while yiur vet is on vacation can help,
What medication was he on? Its possible just needs an extended dose. I myself hsve 7 sick birds, with Chlamydia. They had a full treatment, then relapsed 10 days later, and niw are halfway through a second round of treatment and doing well so far.
For my sickest birds I had to hand feed bsby bird formula. Most liked it and took eagerly if it was warm and thick like greek yogurt almost to thick to draw up in a syringe.
If your bird is eating a seed only diet, then he will be low on many needed nutrients, like Vitamin A and as above calcium. These effect the respiratory system and immune system.
As baby bird formula is nutritionally complete. Feeding some as nutritional support might help.
I suggest to aldo feed some Greek yogurt with Acidophilus as one of the live cultures. It also will provide some calcium and vitamin D. My burds will link it off my finger. I think a half teaspoon to a teaspoon is good. You can do daily for at least week since he has been on antibiotics. If the restart the antibiotics then continue daily until he finishes them. And then a couple of times a week. I have and am doing this with great success. It helped mine who developed loose stool from the antibiotics.
Warmth is very important for sick birds. 80-90 f . I use a radiant heat panel called tge sweeter heater. There are other similar options. Do not use a heat lamp, nasty things that lead to so many burned birds.
Warm cooked sweet potatoes are a great source of vitamin A, many parrots will eat. Also many parrots like cooked oatmeal, and you can grind up pellets dampen thrm with a small amount of apple juice and mix into the oatmeal. I also suggest a little boiled egg, has vitamin B and E, and protein .
Regardless of him being overweight, birds loose weight when sick. You dont want him to loose weight that way. So have a kitchen scale abd track his weight daily.
I would like to see him and his poop. A quick way to share a picture is click go advanced under text box, click paperclip choose oic, hut upload on far right, when loaded close window and post. Pic attach as thumbnail under text.
This is good info for sick burds
https://www.littlecrittersvet.com/sick-birds.pml
From above article
" African Greys: Are often subject to calcium deficiency and will require higher calcium content in their diet then other birds. The best way to provide this is by providing calcium rich foods such as cheeses and yogurt in moderation. Almonds are also a good source of calcium for birds."
I also add you can feed the boiled egg with a little crushed shell. My birds will eat the egg shell sometimes, abd us a source of calcium
