He can always change his willbefore he dies unless it is expressed to be a mutual will.
The sort of trust you describe is called a precatory trust. It might be cheaper, wiser and friendlier just to spit in her face.
It is dangerous ground. Such an arrangement invites an expensive challenge by the widow under the Inheritance (provision for Family and Dependants) Act 1975. If there is an awful lot of money it might be worth it. If not, it is an invitation to provide a few pensions for lawyers.
It is a golden rule for those who wish to be remembered well. that if you have money to leave you do not try to extend your controlling instincts beyond the grave without exceptionally strong reason.