I think your shadow shot idea works really well, particularly with the colours your son is wearing to contrast against them.
A couple of things from my perspective. You've used a fairly wide angle shot which creates a deep depth of field. I don't know that the background adds anything to your son and his shadow so I'd be inclined to extend your focal length to give you a shallower depth of field (I'm assuming f/4.5 is as fast as your lens will open up to). There's also a pole in the background just beside your son's head that's a bit of a distraction to.
Also, a lot of the modern cameras are able to go to quite a high ISO - over a 1000 without affecting the quality of the image too much. I'm pretty sure most modern DSLRs will comfortably handle ISO 800. I regularly shoot up to ISO 1600 for portraits with my Nikon D7000, and up to 6400 for sports and action shots so don't be afraid of bumping up the ISO if you need more light.
Hope this helps
