If he's not too fussed about names, Tesco is selling a hydration back-pack for £5.50 (ish). I bought one for a spare resevoir. It's got a 2 litre in it. Not a bad fit either. It's got chest and waist straps and a big zipper compartment.
Evans are doing the Camelback Scudo bargainously cheap at the mo. 3l bladder and loads of carrying space, they're bmade from very lightweight ripstop so probably not suited to day in day out use.