£20 a day after rent and bills is the norm amongst the students I know.

You must have a very limited circle, of very rich students.
A more "normal" (or typical) amount on most of these threads seems to be around £50pw, after accommodation (which includes bills). Remember that is for about 40 weeks a year, not 52.
There is ALWAYS a real variety on all these threads (and similar ones on WIWIKAU), but £140 a week after bills is a LOT of spending power for anyone.
OP - the biggest spend is accommodation, and that is usually the one you can do least about, particularly if they have to go to London. At least, in London, there are London weightings, which you don't get in other expensive (for accommodation) areas like Bristol. Whereas most Universities have quite a range of different priced accommodation (so you can choose to have shared bathrooms, and to have older accommodation or 'not on campus' accommodation and to self cater to keep costs down) you have to look at each individual University website to see how they allocate that accommodation. Some places are great, but at others, people don't always get allocated what they would prefer.
IF he is organised and willing, then people can also pick up a couple of shifts whilst having those contact hours - particularly, as you say he has been used to working whilst at school. My younger 2 did 8.30 - 4.30 at their 6th form, and had PT jobs as well.