Diffusers can wait. They don't do much to begin with, and only make a noticeable difference in a limited range of situations.
but in the mean time try out bouncing your flash! Your photos will come out much better (less harsh!) than shooting the flash straight on.
Bounce flash. yes. Learn this.
I also have another suggestion for you. I work with kids, and occasionally take photos for our website and newsletter (with photo consents from the parents, of course). A lot of younger kids are just like your son. What I'll do is hold my camera at waist level, set to continuous, and take shots whenever you think he's doing something that might make a good photo. It can take a little practice to get the angle you need to hold the camera at to get him in the frame, but I've had some great results. You'll learn how to hold it to get nice, level shots, but you can also get some great at-an-angle shots! It's a less disruptive technique that still allows you to be right beside them. Since you have a 60d, you can use the LCD if you want, but the more attention you pay to the camera, the more attention he will pay to the camera. I'd try this out with your 50mm lens to start.
Again, this is a good idea. When taking photos of kids, ALWAYS get down to their level. Sit down, kneel, do something that gets you down there.