I think the fuji finepix S5700 is an ideal starter camera for those who don't have the budget to go straight to a DSLR (although the Nikon D40 is now available for £299 on the hight street
). The S5700 has all the levels of control that you would want to see if you were learning such as the different exposure modes (Aperture Priority, Shutter Priority & Manual) allowing your Son to learn about how to control the light. It also has a reasonable Zoom range 38 - 380mm (in 35mm equivalent terms) so the inability to swap lenses won't be a major issue.
The only problems are likely to be a pretty slow shutter lag (the time it takes between pushing the button and taking the picture) and a lowly 4 second minimum shutter speed (so long night time shots are out). However, the shutter lag is only a real problem when shooting fast moving action e.g. sports or moving wildlife, and the limited shutter speed is only a problem if your son want to do that type of photography. It is also missing an external flash connection which can be useful if your Son gets really into the game and wants to try out studio flash work (if he has access to a studio set up at school or college).
FYI a review is available
here
It runs on 4 AA batteries so budget for a couple of sets of 2500 mAH Ni-MH rechargeables from
7dayshop.com and fast charger, and as it takes xD as well as SD & SDHC cards the SD variety would be the better option as it is a more standard - I would recommend the SanDisk Ultra II or Extreme III varieties (2GB SanDisk Extreme III SDs are less that £16 at the moment)
As for your choice of supplier (Pixmania) I know of quite a few people who have had problems with them (long delays in receiving and poor communication) so you might want to do a bit of research into who you buy from.