Having studied Psychology at A-level and a little bit at Manchester Uni, I would say that it does have a fair amount of biology and maths. For eg to get in to Manchester uni, it is not compulsory to have done A-level or equalivent maths or sciences.However it does put your brother at an advantage. Psychology involves a lot of research and experiments and therfore maths/statistics comes useful.
Also, bear in mind other unis might be different,every uni has different entry requirements.
Any other questions, feel free to ask. All the best to your brother