Your levels of ammonia, nitrite, nitrite & actual ph would really help here. I suspect your fish is suffering from toxic water conditions combined with an inadequate diet. How big is this tank? Start changing 50% of his water daily with temperature matched, properly conditioned water until you can invest in a good liquid test kit (such as API freshwater master test kit). Once you are able to monitor his water conditions on a daily basis, this schedule can be adjusted based on the size of the tank. What have you been feeding him? Is he still eating? Some more information will help us to help you better!