From hand shape with his arms crossed over like that it’s the thumb tucked under. Sure makes it look like 6 fingers though.
I’m not sure what you’re talking about. The thumb of his left hand would be at the BACK (under his right elbow). What you are seeing as the thumb “tucked under” is just the meat of the outside of the left hand (the part you would strike with if you do a karate chop) bunching up (happens as you get older, and not just on your hands :-)
It’s still a crappy portrait, and I think it looks like a whole bunch of marijuana leaves around him...