I read somewhere about adding/diluting apple cider vinegar in his water and keep changing it, but I wasn't sure. Would that be similar to the water supplements/medicine you can buy over counter at Petsmart? Those have helped in the past. (In November he lost his companion bird, and had a cold himself, but recovered, and he isn't showing any of those 'sickness' signs like he did then...)
I tried to take a photo of him doing this, but now of course he hasn't done it for the last hour

Thank in advance for any tips,
~Misty