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!