Not to worry, it’s a common mistake. Not all species have the tell-tale spur but many do. It’s unfortunate however that many of our more helpful members don’t recognize the differences between the sexes (and even species sometimes) so you are not getting the help you need or have been lead down the wrong path concerning your pet. I can’t definitively say what is wrong with your chameleon but filling out the “how to ask for help” form might be prudent. In many cases the Furcifer oustaleti are wild caught so you may be dealing with the after effects of the importation process or he is settling into captivity and being dominant (attitude).That said, when the seasons change in the fall appetite does decrease so that may answer that question. The Oustalets come from areas that have long rainy periods so make sure he gets plenty of water for hydration. Obviously if you are concerned a trip to the vet (with a fresh poo sample if you can get one) might be prudent.
For the sake of education, both for you and people reading this thread, the Furcifer oustaleti can be easily sexed by the large hemipenal bulge on the males and lack thereof on the females. Males are generally slightly bigger, have a larger casque (top of head) and tend to be red/brown while the females are primarily greenish.
Good luck!