He’ll need to get a UK phone number - literally no one is going to call a US number because it’s so expensive. Get the phone unlocked (so he can put a SIM card in it from any network). I’d recommend giffgaff, which does very cheap deals on a rolling 30 day basis, which avoids any problems with a 12 / 18 / 24 month contract. Giffgaff is very popular with students.
If he has any plans to see other parts of the UK while he’s there, I’d strongly recommend getting a 16-25 railcard, which gives a third off all rail fares, and it will easily pay for itself. There is a coachcard equivalent, but that’s only valid with one company (National Express, which is a bit like Greyhound, minus the stigma) and coaches take substantially longer. Trains in the UK are generally very good - if you want to see times and fares, see www.nationalrail.co.uk
On the subject of travel, he’ll want to buy a bicycle when he’s in Oxford. It’s the main mode of transport for most students in Oxford (and many others e.g. 17% of working people in Oxford commute by bike), and with good reason. I’d suggest buying a reasonable second hand bike and selling it on when he leaves.
This is a good guide to UK student bank accounts http://www.moneysavingexpert.com/students/Student-bank-account (some will even give you a free railcard).