Ha ha - holding his stomach in! Didn't Lynn Redgrave say he looked like a "pear" in his later years? Oh Charlot...
He's definitely sucking in! Hey, we all do it. He's still pretty slim at this point actually. There's a funny quote in the "Interviews" book from around 1940 where Charlie is talking about his girth and says that it's a pitiful time for old actors when they have to start holding in their stomachs for still photos--and sometimes they forget.
Oh those trees!!!
He's still perky too!
I didn't even notice he was holding his stomach in until I read the comments. Personally I'm an arm person (not too much muscle but a little) and those look fine to me.