My heart goes out to you with Patcho, dealing with illness is never easy.
I'm not an expert here, but my BFA Sydney has been chronically ill since I got him several years ago. He's anywhere between 35-45 years old, but I have no way of knowing for sure, this is just based on information given to me when I got him.
He has chronic air sac problems (seems to almost "leak" into one particular sac in his shoulder area) and bacterial infections. He may also have liver issues due to being on a seed diet most of his life, but I haven't gotten him rechecked in a while for that, which is entirely my fault.
For medications, the only way I've been able to get him to take it, is by spoon. I mix the dosage in unsweetened cinnamon apple sauce, but in your case you might want to use sweetened for calories. It's only a tablespoon or so, so it wouldn't be much.
For dietary support, I make egg bread, which is essentially cornbread with extra eggs added, usually 4-5 depending on how much I'm making, and adding a few scoops of parrot formula for additional calories, vitamins and minerals. I also sprinkle Prime supplement on his fresh food a few times a week, larger selection of vitamins and minerals, as well as probiotics. He also gets a capfull of apple cider vinegar in his water a few times a week, more often in the warmer months.
I don't use a heat lamp for him, but this year I'm considering a heated perch or heated panel for him. If I need it though, I do have a ceramic heat bulb and a standard heat lamp with a dome that has a dimmer switch but I haven't needed to use it for him.
I am skeptical of some homeopathic treatments, but I am considering milk thistle for him, but I may go with the actual seeds rather than a formulated liquid. Ask your vet about Emer-aid powder as well, you have to get it through a vet as it's not available to the public, but it helped one of my birds last longer than she should have.
https://emeraid.com/emeraid-omnivore/
It's much more easily absorbable than a lot of critical care diets, but ideally it's syringe fed or tube fed.
I'm sure there's things I'm forgetting, but this is all just from my personal experience and I hope that there's something that you can use to Patcho's benefit.