if hes still little (based on bloodworms and krill as foods) he could have swallowed too much air and not know how to burp it up, does he seem to have trouble swimming down or is he floating by choice? if they seem stuck floating move them to an isolation tub where the water can be low enough to just cover them, this should help them calm down and burp out the air.
if hes over 4 inches long my suggestion is to try other foods, those foods are ok for babies but dont have the nutrition bigger axolotls need. Pellets can be hard to get your 'lotl to switch to at first but are pretty easy overall, earthworms are the best food for them but can be hard to keep in stock so sometimes i freeze them for later.