tbh, they are all the same thing with different packaging. if that is something that is important to you, then choose what you like from there. otherwise, go for the cheapest.
i have long thick hair, down to my bra strap, and own the bigger fatter version of the ghd. i cant actually see them on the website, but is also the mark iv. fabulous imo. but, if your son has short hair, then you probly dont want the fatter/broader ones, andthe normal size should be sufficient. the prices, if they are to be believed, are bargains.
hercules, depends on your hair. if its fine, andnot much of it, they might be ok. my friend has a pair of 12 pound ones that work perfectly fine for her. but for me, nothing but ghd's work. the rest is all just a waste of money and time.
GHD lover here too. Well worth the money as my hair is incredibly thick. It does take some time to get used to using them though. I have the thin version as I find them easier to use. They also get the shorter hair - if your DS ever decides to cut his hair a bit shorter.