Silverapples did your DD study in London? The London loan is higher to take into account the higher cost of living.
Copycat Firstly, as your DS is an adult, he needs to do all the contacting of the university. They may refuse to discuss it with you. If he has additional needs that make this hard, he needs to communicate this to to the university and say that they are allowed to discuss it with you. Otherwise, it should be him doing all the contacting.
£5011 is a lot of money, but chances are a lot of the students put down the cheaper halls, and they will currently be full. Your other options are to consider private halls (which may be no cheaper) or a room in a shared house (which will be cheaper but more complicated). You can of course reject the offer and explore these options, but the university is not going to have space for everyone in the cheapest halls.
Does your DS have any savings, and can you afford to support him at all? If so, it may be best for him to take up the place at the more expensive hall, and wait for a space to become available at a cheaper one- this happens due to students dropping out or wanting to swap flats. This can happen quite quickly.
Otherwise, I would suggest looking at finding him a room in a shared house, and getting him let out of the contract. The university may be able to advise of spaces in houses with other freshers in, or at least give a list of approved accomadation.