Hi there, you are through to Dr. Jake. Im a UK based vet. i sorry to hear of your concerns with your fish, Kevin.
theres ia few things that could be the reason for Kevins signs.
Initially getting a water test kit is a good starting point as a large proportion of problems we see with fish are related to water quality. water quality is very important in aquarium fish, small changes in ammonia, nitrates and nitrites can lead to disease. i would recommend getting a full water testing kit from your local tropical fish shop to check these are correct, regular water changes can help combat rising levels of these water toxins.
a lack of swimming or altered swimming can be due to buoyancy problems which we see commonly in fish, they can have reduced movement, swim with their head down, up or can be found upside down.
one of the most common causes is due to an infection within the body , the bacteria cases excess gas production and alters the fishes ability to float and swim. This gas can be produced in the swim bladder or also in the gastrointestinal tract. this is a condition that can potentially be treated with antibiotics and also removal of the gas with a needle if required though there is a possibility it will re occur
another reason could be pressure being put on the swim bladder by other structures in the abdomen, tumours can cause this - I would expect this to be less likely in this case as the fish is young though we do see tumours in young fish unfortunately. this can potentially be diagnosed with imaging (x rays and ultrasound)
another cause of abnormal floating and swimming is neurological conditions - tumours in the brain or neurological system or degeneration of the nerves.
if he was found stuck, there is a possibility that he may have caused internal trauma affecting his bouyancy which may settle with time.
diagnosis and treatment in fish can be difficult and sometimes we dont get a full diagnosis so please be aware of this
for further investigation i would recommend finding a vet who is happy to treat fish for examination and medications if required
a list of fish friendly vets can be found here:
if they are too far away from you then still contact your local vets as they may still be able to treat your fish with some guidance from seeking advice from a fish vet.
i hope all this information helps