If he is in need of care and if he has money, he should pay for it himself.
If he is not looking after himself, could a relative apply to have him made a ward of court. Then a court appointed person,probably a relative(?), would be able to use his money to look after the old man.
A will is not a contract. The family could look after this man and he could still leave the money to the religious order. And vice versa, the religious order could look after the man and he could leave his money to his family.
Is the religious order in the business of looking after elderly people? If so, could he not pay them for his care?