Yes, he's holding his guitar right, and I'm not sure what you mean about where your thumb is, it should fit naturally where it's best for you.
some books say the left arm elbow should be near one of the corners of the guitar, i find this weird
well i hate to break it to you, but unless you're playing classical music, there is no 'correct' approach to do things, but people generally keep their thumbs in the center of the back of the neck, and it tends to be almost perpendicular.

however, people may shift their hands to the baseball bat position (thumb over neck) for leverage when bending, but it is not recommended to play in the baseball bat position for other reasons