Exhaust his options. Make him marry someone else in your life (right now) …after your death, she (2nd wife) will make sure he don’t get married again. Bright side is that you gotta chose his new wife. Also while you are alive, make his married life hell (which I am sure you are working on). He will do taubah and will not marry again anyway
PS: ON serious note, I dont agree with this. I’d like my wife to get marry again after my death. While I am dead I dont want (and expect) her to live like a living dead. Beside this its against nature and I’d say even Islam. If this was such a good thing, Islam would have asked for it (for not marrying again). This point of view has come from Hindu religion where they want he widow to live like a dead women/man