The phone can be unlocked to work with any network.
I've always had good PAYG service from T-mobile, who have merged with Orange.
Current T-Mobile Talk plan: 20 per min, 12p per text, but top up the credit by £10 and you get 100 free mins thrown in for a month.
Current T-Mobile Text plan: 25 per min, 10p per text, but top up the credit by £10 and you get unlimited texts for the month.
Averaged over a year, I suspect all the networks are similarly priced when you take all the special offers and gimmicks into account.
A contract should be cheaper if your son will make A LOT of calls or texts, but PAYG is fine for light users like me, and it's nice not to worry if my income suddenly drops to zero.