YABU to pressure him. It has to be his decision.
But you can support him to make an informed decision, where he weighs up the tradeoffs. He needs to look at the full costs of living over every year of the course. Including rent, energy, food, laundry, Wi-Fi, clothes/shoes, transport, books, cost of hobbies/sports, socialising.
Bear in mind average rent for a room in a shared house/flat is London is just short of £1k a month, exclusive of bills. In Oxford, just over £800pcm.
Once he knows how much he’ll realistically need pcm, he then needs to weigh this against his loan, any likely part-time earnings or savings from working holidays and what you can offer.
Then, he’ll need to factor in time. For day-to-day chores like cooking and laundry, as well as studying, working, hobbies/socialising, travel. Long commutes can be costly in both money and time, as well as restrictive, e.g. staying late in library or socialising.
My DC lives at home, as can’t afford to rent separately and even then has to be careful with money, even with p/t work. The eldest tried it and ended up in debt.
How much parent/s can help depends on family circumstances, wealth, income and housing costs. If parents own their home mortgage-free, it’s a big difference to those renting. Retired parents or those on low incomes can’t always be tenancy guarantors.