It could be any number of things-including coccidiosis, and IF he is not eating be sure to feed him, because that will bring him down quickly.
Be sure to isolate and put him on heat, and keep him out of drafts. Make sure to feed him and that he drinks.
How does the poop look? How does the inside of his beak look, any lesions? Does he have any other symptoms?
Since he is still so young, get him some good probiotics and put a drop of organic apple cider vinegar in his water bowl (1 tablesp to a gallon) Youngsters can easily deplete themselves of good gut bacteria when they are stressed, and then the bad bacteria has a chance to populate.
Worms, Coccidiosis and Trichomonas are all common issues. If you treat the
bird w/Trimethoprim Sulpha, you will automatically treat for both Coccidiosis
and anaerobic bacterial infections simultaneously. Trimethoprim/Sulpha may also be combined w/Metronidazole or Ronidazole (Trichomonas medications) so the bird may be treated simultaneously for several issues. When a sick bird is not self feeding, it is imperative that you feed the bird so that the bird will not'spend down' its' reserves.
Here's a link to the Pigeon Supply houses from the Resource section in
the main menu:
It would also be good for you to get a broad spectrum wormer and one such
as Scatt or Ivermectin that will also treat for feather parasites that feed off
the blood of the bird. Having Baytril on hand is also advisable (Siegels has the pill form.