Definitley follow what devilish said about water conditions. That is #1 priority. I agree with adding some salt too, maybe about 1 tblsp per 5 gallons. If he looks like hes getting holes or little depressions on his head or along his lateral line than he may have hexamita, or head in the hole disease. Also look for a white stringy material coming out of his anus or floating around in your tank. One can occur without the other. I had this with my flowerhorn after getting her from bad living conditions.
You may want to proactively give him an antiparasitic food just to be on the safe side. They are very safe and pretty cheap.
Thats all I can say, except watch him and keep that water pristine!!