www.lefthandedgolf.co.uk look no further!
The 'US Kids' stuff is very good, about as good as kids golf clubs get.
If you decide to phone them up and order, they will be able to instantly get the correct size clubs if you can tell them your son's height and the wrist to floor measurement.
This is done by getting him to stand up straight "like a soldier!" and measuring in inches the distance from the crease in the wrist, basically where the hand meets the forearm where it bends. (Measure both left and right and take the average, they will probably be different). I think he might notice you measuring him (!) so you may have to pretend its for a new coat/trousers/schooluniform or something.
If you are able, a little bit more than 50 quid might be needed with postage etc, but I guess when you're both out on the course together and he's kicking your arse in a couple of years time you'll know it was worth it!!!