Sorry to learn of this situation, he sounds a complete dickhead.
Have you considered consulting the Citizens Advice Bureau? Confidential, impartial advice and wouldn't be obliged to act on your information or intervene.
Edit: Apart from the amorality of his behaviour (he doesn't seem to know what moral behaviour is), he's using threatening and coercive behaviour to force your mother to dip into her estate finds, thereby ultimately reducing their value and your potential inheritance. It's also obviously adversely affecting her health.
Good luck.
Regards?
Chris