The money must either have been in his bank account (or a parent's, I think?) in full for 28 days before he applies, or he must have an approved US loan that will cover the amount in full, and evidence of that loan (i.e. a loan letter).
Most people here on UK-Y seem to fund their studies using US student or private loans - I've seen Sallie Mae and FAFSA mentioned here before. You just need to check that the UK university is FAFSA-registered and will accept the US funding, as it is distributed through the UK university.
He needs to show 1 full year of tuition, plus the minimum living cost amount for the area he is living in. If he will be living/studying in Inner London, he needs £9,180, and if he will not be in Inner London, it's £7,380.