If your nephew likes them then that's great. *Personally* though as a guitar player and photographer...
Left to right:
1 Best of the set - the pose is a little bit 'here is a guitar' but it's OK, but the lighting on his face works well.
2 Looks un-natural with his hand at an awkward angle and the neck too far away. Good lightig on the face again.
3 Good idea, static head needs to be crisper to differentiate from blurry face.
4 Just a bit too obviously posed, although it's an easy-going photo.
For the shots where he's fingering chords, the guitar really needs to be on a strap to place the neck sensibly for the hands to look right. A re-visit with the same lighting used for 1 & 2 but where he's really digging into the guitar would make for more exciting poses. I hope that's useful, rather than coming over as negative.