If he's been eating in captivity, it isn't captivity stress that's causing him not to eat so don't return him to the wild because of that. If he's acting healthy otherwise he's probably just sensing the season change and going off food. Usually I don't start to worry unless mine have gone off food for at least a month. If you don't already have a heater or heat lamp for him, getting his temps up will probably get him interested in food again.