Is he visibly thinner? Frogs can sense the change in barometric pressure as winter approaches. Some of them eat significantly less for a while. As long as temp and humidity are good and the frog looks healthy I wouldn't get too concerned. Got any pics of your frog?
I wondered about that. The room he's in has gotten colder and the days are getting shorter. I do maintain the tank at a constant healthy temperature but I'm sure he can sense the change. My hermit crab is the same, much less active in winter.

I don't have a recent photo of the frog, I'll get one in the next day or two. I handled him a few days ago and try not to disturb him too often.