Hmm. Looks a bit bloaty to me and his vent is sticking out. How is his appetite? Maybe you are just overfeeding him and he is constipated, I would try some peas first and see if that laxative will help him. Also, does he poop long white strands? It could be internal parasites if that is the case. There should be a sticky on how to treat hexamita with either epsom salts or metronidazole at the main part of this subforum. I think there is also a medicine called flowerhorn clear that is supposed to work pretty good.
IIRC flowerhorns are pretty susceptible to getting it for whatever reason (I think because of the hybridization), and if it was in a tank with some gt's it may have spread to him from them, gt's are super susceptible also. I lost a baby gt a while ago from hex. You will want to knock it out quickly before he stops eating or treating it will be way more difficult.
Is the tank fully cycled etc etc? Keep up on water changes for sure.