England and Wales seem to lag behind Scotland (where the Adults With Incapacity ( Scotland ) Act 2000 applies), so there appears to be no directly relevant legislation. The Mental Capacity Act 2005 might have to be used to appoint a 'Deputy' (rather than a 'guardian') to act in certain ways upon behalf of the son. If so, I can see no way that such an appointment could be made under the terms of a will (or in any other way, prior to the death of the mother), as it requires a court order. However the mother could indicate in her will what her preference would be in such a matter.
The services of a solicitor might be required here but, as a starting point, it could be worth phoning the MIND Legal Line on 0300 466 6463 (Monday to Friday, 9.00 am to 5.00 pm). Calls are charged at the same rate as those to 01 or 02 numbers (which, for people who've got 'included minutes' on a mobile phone contract or PAYG 'bundle', or a landline package which gives free daytime calls, would mean that it's a free call). Alternatively send an email to
[email protected].
The Office of the Public Guardian might also be able to provide advice. Call 0300 456 0300 (Monday–Friday, 9am-5pm, except Wednesday 10am-5pm). Once again, calls are charged at the same rate as those to 01 or 02 numbers. Or try an email to
[email protected]