I agree it may just be the sickness that they come with (I forgot what its called).. It can take months before it actually affects them but sometimes it happens earlier..
Or it may be something else.. But I don't know what else it could be.
Maybe feed him a de-shelled pea? (I think that's what helps with bloating.. ) even though I don't think he is.
Did you cycle the tank with him in it? And I know pH isn't much to worry about but maybe your pH is to low for him. Shooting in the dark of course I'm not a marine biologist lol. Pea might help if he still swims for food. Maybe increase the oxygen in the tank.