Go out there prior to the shoot, take your camera and just start shooting pictures, that will enable you to start seeing your background. There's a huge difference between what our eye sees and what happens when we frame it in a camera. It will also help you to see what you'll have to avoid. Remember to try to keep it clean, remove any debris from the foreground and avoid weeds and trees sticking out of his head.
I can see why he wants the background. I would attempt one with him sitting with his back against the wall with the "mountain" behind him. Oh and remember, to blur your background, move him forward from the background and the get up close to him. Good luck and don't stress it, you'll do fine

There's been plenty of talk about the hardware (all good advice) but very little about the location.

I recommend against using some of the more colorfully painted sections as a backdrop. I think they will be confusing to the eye and distract from what should be the focus; your grandson.

I'm imagining he'll want to get the brightly colored work as a backdrop but pick location where there's little or no graffiti and do a number of shots there too just to hedge your bets. Come at the background at an angle. Try and find the more abstract works to reduce distraction.

Most of all, shoot and have fun!

I will go along with the advise that JR1 gave in shooting in aperture priority. Set your ISO at 200 to 400 so you will not get noise. Then set your aperture for the depth of field affect you want. Let the camera set the shutter speed (of course watch it so it doesn't go to low). If you go manual and set the shutter speed yourself by using the in camera exposure meter it will be the same as the camera setting it. You will need to check and make sure the looks right on the monitor and adjust if need be. Your would do the same if you set it in manual. Why mess with this when you should be concentrating on composing your pictures? You paid a lot of money for a camera with three exposure modes use them. In the exposure triangle (shutter speed - aperture- ISO} you can only pick two to get the effect you want. The other one can only be one setting wither set by the camera or you. Of course there are things that the camera can't meter like the moon or in the studio with lights but not many. I know that it's fun to say I shoot in manual but very few pros do in this type of shoot. - Dave

If you are not used to shooting manual stick to Ap priority so you can chose what is in focus, I think he is right the backgrounds are great, BUT remind him he is the "focus" not the walls.

IN 1.... I would have him AT THIS angle shown here sitting in it knees up looking past NOT AT the camera

in 2. I would go for a daring wide angle LONG shot with him laying on his left side top (head end" up shooting widest angle from his feet along him and the wall, he will be elongated and it should be dramatic, try to get higher than him, YOU about 4' in front of the wall looking slightly down to exclude rubbish over the wall

YES I KNOW these will get comments, but I wanted to something fast

Well the lens is ideal, I can't seam to get the link to work, GREAT to see someone with a reflector, gold for warmth, though you can always adjust the kelvin temperature after in photoshop.

Focal length NO

Ah, link finally worked, graffiti is that right !

Personally I would shoot from leaning on walls and sharp with the subject closer to the camera at a diagonal with the wall tapering off to the right almost side on

Similar to the 24mm shot here but with a deeper view of the wall

http://www.mcpactions.com/blog/2010/07/21/the-ideal-focal-length-for-portraiture-a-photographers-experiment/

And here

http://photo.tutsplus.com/tutorials/photography-fundamentals/exploring-how-focal-length-affects-images/