How do you tell if a lawnmower blenny is skinny?

I you just got him give him some time to get use to his surroundings. But from what I can see he doesn't look to shabby

 

Did you try rubberbanding a piece of nori to a small piece of rock and dropping it near where he hangs out?

 

Thanks. I know he isnt the worst, but I wasnt sure if he was still bad. I am not a big fan of having FAT/"well fed" fish, so I am very ok with him not ever looking super round, I just want to make sure he is atleast at a decent weight.

I tried one piece attached to the wall above the pump (his favorite spot) and one on a rock close to his second favorite place to hang out. It was a no go for those.