Holy fat blenny! now that that's out of the way...
What are you feeding him? Blennies need a balanced diet that consists mainly of plant matter (well most species do). If he is consuming to much fat or proteins he could be backed up. Their digestive track is not really use to breaking down large amounts of animal proteins quickly.
I have an orange spot blenny that had a pot belly for a while, until I began limiting his food to mostly algae grazing pellets. He then slicked down and began looking healthy again.
If he is still eating normally I wouldn't be concerned that this is a bacterial infection. However, this doesn't mean that it couldn't turn into something later down the road.
Hope your fat fellow remains in good health!