First off, you open the image in the Gimp (I'm afraid I don't use Macs, so you'll have to figure that out. It's the equivalent of right-click > Open With...), then you go to Image, Scale Image. You'll only be interested in the two numbers at the top; they will probably be between 2,000 and 4,500 depending on your son's camera. You can enter the numbers yourself, or you can hold down one of the buttons until you're happy. A word of advice, though: Make sure the top two buttons are "locked" (there's a little icon to the side of them). This makes sure that, when the image is resized, it doesn't look squashed. Resize the image to a level that you're happy with (if you think they're going to print the photos, I would recommend a resolution of 1,200-1,500 pixels in the largest dimension). Click OK.
After that, the compression. Go into File, then click Save As. Rename the file as you wish, but make sure the file suffix is .JPG. Check to see if the file destination is right (I have found the Gimp's file saving windows a bit confusing at times), then click OK. It will then ask you about compression. Like Big Mike said, the more compression, the smaller the file size, but the worse the quality. I find a number of about 90 to be good. Then click Save and check it looks good.
Now, I shall admit: This is not very good if you want to process 200 photographs, but for one-offs, it's fine. Enjoy, and best of luck in the competition!