He would look sick if he had not eaten or drank anything in a few days. If I were you, I would get him out of there and do a thorough body check. Check his feet, vent, his breast area, look at the face, eyes, nostrils, and injuries, etc...Look for anything that might be ailing him. Get everybody wormed if you have not already done so.
Then I would let him out with the others to make sure he is not being picked on and is hiding. If this is the case, you may need to separate out the aggressive ones or find a bird that gets along with him and keep them separate.
If he is just truly shy, then in his best interest, if he were my bird, I would put him in a separate area with a mate, put in some natural branches for cover and work with him, (treats and attention) and see if he will come around.