Bullfrog slowing down for the Winter?

[Gnag the nameless]

In the past few months, my larger, nearly five inch snout to vent male American Bullfrog (Rana Catesbeiana) Gnag has begun to eat less and less. I've already done various thorough checks for any signs of illness, and found absolutely none other than the lessened eating. He is also less active, but will occasionally hop and swim around a bit one night, then remain near motionless for the next few nights. He does not appear to be losing any weight, and he is quite the plump (and muscled) frog.

Now, he is hardly eating every few days. I've gone ahead and done some research, and apparently it is natural for adult American Bullfrogs to slow down and/or hibernate during the winter.

However, Gnag's tank is 90% aquatic - the only landmass is on top of a piece of driftwood that makes up his shelter and atop a small clump of moss for him to sit on in the corner of his tank. He's seemed to become less and less active, and I'm wondering whether I should:

1. Just leave him be, offering crickets daily and keeping an eye on him.

2. Or put him in a large plastic box with a deep layer of mud on the bottom and let him burrow down to hibernate.

I've read that you don't necessarily have to hibernate American Bullfrogs as long as you keep the room they're in relatively warm. Having absolutely no experience with hibernating animals, I'd rather avoid having to do it. The room Gnag's tank is in gets to 16 Celsius at the least, and 23 Celsius at the most (room temperature most of the time). Also, I could purchase a few Hornworms and set them aside for whenever he gets hungry, so when he does eat he gets the optimum meal.

Any suggestions as for what I should do? And has anyone else experienced this before?