You should probably stop the antibiotic. One of the biggest drawbacks to antibiotics is that they don't differentiate between good bacteria and bad. A bird's system (as well as mammals') requires a balance of bacteria to work in harmony as digestive aids and even immune system boosters.

When antibiotics are absolutely necessary - I'm all for them, but unless there's a specific infection diagnosed, you may be impeding the bird's natural defenses and making his body resistent to the antibiotics (which will be devastating if and when he really needs them).

The symptoms you describe are some commonalities in birds suffering a deficiency in certain vitamins and/or when fat is not properly processed by the liver and the potential for disease rises.

A large number of birds on seed only diets have both a deficiency because seeds are not a complete diet, and they're extremely high in fat.

What many owners don't realize is that even in the wild, our companions don't eat only seeds and they're constantly flying and foraging, burning off those fat calories throughout the day.

Unfortunately, marketing ploys represent seeds as a ‘complete diet' or ‘fortified', along with other questionable claims that aren't quite true.

If he's been on a predominantly pelleted diet with supplements of fresh foods daily, the symptoms are more perplexing. If there's any possibility he's a "she", the threat of this being egg binding or dystocia makes it an emergency situation that needs to be seen right away.

Either way, something going on this long should be evaluated by a vet.

They need to do a physical exam. That means hands on, feeling the chest area, peering into the mouth with a well placed flashlight and lifting the tail feathers to examine the vent.

This exam should also include any one or more of the following: Blood tests, gram stains/cultures, x-rays, even oral/crop/tracheal swabs and so on.

If the examining vet doesn't perform a hands on exam, or worse, leaves your bird in their cage or carrier, leave immediately. This is not the vet for you or your bird.

Find an avian vet near you

http://aav.org/vet-lookup

When it comes to a bird like this, many local vets will be able to see him and are well experienced in bird care, though an avian vet is desired, it's more important to have him seen than to find a DVAM.

You're doing the right things - keeping him in a temp controlled environment, calm and well fed/hydrated is vital. Offer him a few drops of children's Pedialyte to help with this and if he'll take a taste of natural (absolutely all natural) yogurt - it might help offset the days of antibiotics and boost his good bacteria.

Putting a heat source into the cage may be necessary since a sick or distressed bird may lose body heat. I prefer a non electric source and use rice socks.

Use a thick, clean sock and fill it ¾ with plain, raw white rice. Knot the end and microwave it for about 1 ½ minutes. Shake it afterwards to distribute the heat and be sure it's not too hot.

You can layer a thick towel on one side of the cage, secure with clothespins out of the reach of the bird's beak - then clip a heating pad over the towel and set on low. Check often to be sure it's not overheating and that the bird isn't gnawing through. A side attachment like this will allow the bird to move closer or away as needed.

If the bird is bottom-bound, install a heating pad under just a portion of the base, again, set on low.

The rice sock option is far safer and more efficient though.

Cover the cage with a thick blanket, leaving all or part of the front door uncovered for better air circulation, to help keep the bird calm by being able to see out and of course so you can check on it regularly just by peeking in. The covering also helps hold some of the heat inside.

I'm very uncomfortable giving the heating pad suggestion because of the potential for the bird to bite through the bars and possibly catch a part of the heating pad. This is just one reason I prefer to avoid electric sources of heat. Another reason is that heating pads can suddenly become faulty and overheat. If ever using an electric source for heating anything in anyway, please be vigilant and constantly double checking carefully.

Please let me know how he makes out