Does he have a water dish that is big enough for him to soak in? The heat and humidity could be bugging him a bit. I know my garters get a bit bothered when it is really hot and humid here...it is important to have a large water dish that he can soak in if he becomes hot. Also, I have noticed that some of my garters do go into their hides and lay around for a few days after a big meal. Some garters do not like the heat more than others. Some of mine basically just lie around on the cool side of the tank when it's really hot. Is he pooping and flicking his tongue?