Could he be dehydrated? Mine had similar symptoms when it got cold outside and heat was coming on more often. I started misting him and he drank
a lot and now eating more.
You might try giving him a warm soak in the morning. I use a bucket with warm water about shoulder high. Mine will usually poop in the bucket in 2 or 3 minutes. I blot dry with a paper towel and put him back in to bask/finish drying. He rarely poops in his tank.