That's not looking good. Can I ask what his diet consists of? If you are feeding live don't. I read you haven't seen him poop even though that in of itself wouln't tell you if it's parasitic or fungal, but if it's eating doesn't poop and has a sunken stomach you might think tapeworm. I would treat the tank with Jungel Parasite Clear or soak some skinned peas in Prazipro about 6 if he's eating these guys eat peas like it's candy and it will clear the digestive tract in combination with the parzi should flush out anything in it's system. While your treating with the meds don't feed it except for the 6 peas as mentioned above. After the course of treatment do a very large water change. Good pellets for GTs IME are NLS Thera +A the extra garlic creates and enviroment in the gut that is unhospitable to IPs or Hakari Bio-Gold+ works also a good quality veggie wafer a few times a week is good for them along with a few skinned peas a week help keep the digestive system clear. If you can get it on NLS either the Thera +A or regular Cichlid/cichlasoma formula it can live on that forever. Hope this helps good luck he's a nice looking fish with def potential.