It is very hard to describe the process of drawing something with out showing you what to do. I would suggest looking online for a tutorial on how to draw one. IT shouldn't be hard to find and you should also look for one on how to scale up drawings.
I would advice looking up online some pictures of dragons and choosing the ones you like the best. Try some sketches out in a notebook before doing the actual wall.
My advice would be to pull up a bunch of images online of dragons and practicing drawing them on paper first. Once you have perfected it I would try it out in pencil on your son's wall. Use simple lines and ten go back in and add the detail! Good Luck!
