Does water quality check out ok? Do you do plenty of water changes, not just to keep nitrates in check but to flush out things like hormones and metabolic waste?
If you feel certain he is getting enough food (they are omnivores btw, so while mine absolutely *loves* Hikari sinking carnivore pellets, he also relishes a piece of cucumber or strip of nori) and that water quality is good, I wouldn't worry...even though it does seem like he should have grown a bit more by now, to me. Mine (I originally had two, that didn't work out forever though since they don't like one another usually) grew very fast, even went through a chubby phase like the pic above, but now that he (I think of him as a he, they are very hard to sex!) is about 8 years, old he's gotten trimmer.