Poor little dude. I hope the medications help. 20g is a light parrotlet. I hope he gains a bit and gets stronger again. Grey poop is scary. I hope the test results come in quickly.
Did they run bloods? Where are they sending the sample? It is best for parrotlets to get blood work done at the same clinic that does the draw. If they need to ship the sample out, the tiny amount of blood a light/weak parrotlet can give up is so small that it will not be viable for long.
Poor little Sprite! Feel better soon little one. I will be thinking of you.