Hi & welcome,
I have had 2 Gray's since July and found that they slowed down as the weather here got cooler ~ early Sept (one also changed color from bright green to gray). They weren't eating as much, but were not as quiet as yours seems to be. I thought they were trying to hibernate, as yours probably is and I've been using a light to provide heat during the day, and turn it off at night. Since then they have perked up a lot. More active and better eating habits. Guess its a choice of letting him sleep, or keeping him awake! Good luck!