lmkl, you're welcome. I wasn't sure about the waxiness so I looked at Amazon's reviews and found this: "The actual floss in these heads is so low quality and not waxed enough that if you have tight gaps between your teeth it's nearly impossible to get this stuff through." That was the only time I saw it mentioned. The dentist told me I have a small mouth last year, and in all that time of using the Reach Access flosser (since I saw the dentist about a year ago), I can count the number of times the floss has broken on one hand. But my teeth might not be as tight as your son's teeth.
Maybe you could try the Reach Access flosser (though it says ages 6 and up, so you would need to help him) and if it doesn't work so well, ask your dentist for something similar?