very nice
i like the colors in this piece, and im also a fan of that guitar. However, i would look at thinning the actual neck of the guitar. Also, the way the neck is drawn it looks like its twisting to face the viewer. i would suggest keeping the same angle on the bottom half of the guitar to make it more believeable.
Also, I think you should center the character so the focus is directly on him. Not that it isn't already, but it would make the character stand out even stronger if he was centered. Mostly because you have a lot of empty space on the right as compared to the left. so just balance it out.
lastly on the guitar, try to add thickness to it, the angle your character is holding it would make it show at least a bit of the thickness of the guitar. at least a little bit of thickness to it if you decide to make the body of the guitar a weapon of some sort, or a blade.
anyways, i like the character, and im definitely a fan of the background thats pretty sweet. good work!