The fact that he's light could definitely be a clue. He might be getting bullied away from the feeder, or he could be fighting an illness of some kind. Either way, if he is underweight he definitely isn't going to be feeling his best and that could definitely explain the change in his behavior. Is he the only one of the flock that feels light like that? Are there any others showing potential respiratory symptoms?
Parasites could be a potential cause of weight loss. Is he showing any signs of mites or lice? Worms? If he hasn't been checked for worms recently, it might be a good idea to send a fecal sample for testing just to make sure.
Offering him some extra protein in the form of egg, canned fish, or something similar would also be a good idea. With my birds, they have always started to regain weight and energy within a week of me offering them hardboiled egg either daily or every other day.
