He definitely has a little chunk to him! Maybe put him on a diet since an exercise plan is probably impossible. If that belly remains after a cut in feeding, then i'd be concerned with something else going on inside.
I wouldn't worry to much. My starry blenny has a HUGE belly and has since he was put in the tank 2 years ago. I have heard more than once a belly on a blenny is a good sign. just my two sense.