It might be an abcess if it is firm. Chicken pus is hard and cheesy, i'd be tempted to try and lance it as far from his actual vent as possible. His picking makes me think he's trying to open it up himself.
Have you squeezed it any, not hard but you know put pressure on it? And is it hard or squishy and is it hot? Sorry so many questions but if it is an abcess lanceing it would help big time.