Have you a preference for network - most of the phones will be locked, and the choice of network may make a difference...
Also, on whether he's likely to want to take lots of pix. If so, then the problem with the cheaper end phones is that they don't all have a slot for a memory card. Some have 10 to 15 MB of memory - OK for VGA photos but the better the quality of the camera image, the more memory will be used per photo.
I have a VGA camera in a Motorola phone bought at Argos for about 30 quid, but that was in pre-Christmas sale a year ago.
It has Bluetooth so can send or receive pix from another phone, but because the Motorola is only VGA it won't show pix taken on my other phone (Sony Ericsson) and in terms of storage, the photos from the other phone are 700 to 800 KB so it would not take many to fill 10 MB of memory in the phone.
Depends also on your budget - saw a couple of phones on the Asda website not long ago which were each 34.99 (not sure if they force you to get a 10 quid top-up on top of that cost).
Both are Vodafone PAYG (so you could then put in an Asda PAYG SIM - 8p/min and 4p/text)
Nokia 2630 - MP3 player and VGA camera
Vodafone 533 - 1.3 Mpixel camera, MP3
Not only does the second have a better camera quality, but checking this page on GSM Arena it can easily get 2 GB of music and photos if you use a microSD memory card (one of the cheapest around).
There may be some others on the Argos site, or Carphone Warehouse www.e2save.com site.
The Vodafone site has the Vodafone 533 for 30 quid (but it says "great value at 40 pounds" so I assume they charge for a top-up).
Once the Voda credit is used up, could then swap to Asda if you want for cheaper calls. (at 8p a minute to mobiles and landlines, Asda is cheaper than daytime rate on a BT line to call a mobile!)