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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...

The first thing I would do in this situation would be to pick him up and give a full health check. A dramatic change in behavior could indicate a possible health issue.