He would have most likely lost weight if he had worms.
If he is not eating/drinking then you might want to consider tube feeding him.
Here is a link to tube feeding - this is an active thread and Kathy (@casportpony ) monitors it so if you have trouble with feeding him she will usually help...
Poor guy.
You may want to worm him. If you have a vet that can perform a fecal float test on a poop sample they can confirm worms or not.
It could also be a bacterial infection of some sort.
Any signs of lice/mites-an infestation can cause weakness and in bad cases anemia, but his feathers are...
How long have you had him?.
Can you post a photo of what he looks like?
If he is new to your property then, it could be Cocci
You may want to make sure it's not a nutritional problem and limit the seeds, pasta, scraps to no more that 10% of his daily intake to see if there is any improvement...
How old is he?
What type of food/treats do you feed?
How does his crop feel?
It's possible that he may have Coccidiosis. Some signs are weakness/lethargy, not eating, diarrhea with mucus or blood, loss of balance, and puffing up.
Corid dosage for Cocci is 1 1/2 teaspoons Corid powder per...