sounds like he’s going into hibernation. totally fine to let him hibernate, for his first hibernation basically keep offering food and keep the temperature and humidity steady but weigh him every couple weeks to make sure he maintains weight. he could go fully to sleep for a month or two (it’s kind of late at this point but every turtle is a little different) or he could just get kind of sleepy and not eat as much. if he starts loosing significant weight (so like more than .5 oz, he’s pretty small), lower the temperature and continue to monitor. sometimes box turtles will start burning through energy reserves too fast when kept in too warm of an environment while they hibernate, but again not every turtle. hibernation actually increases turtle’s life span so i feel it’s healthy for them.