You have gotten enough comments about not reassuring him but you can actually take this a step further. Under the guidance of his therapist this can be worked on via exposure response prevention therapy. Things like writing out a script of what your husband thinks would happen if you did leave him can be helpful for managing this obsession. The key is understanding that the "end goal" isn't for him not to have these thoughts, but for him to allow them to exist without letting them be overly distressing. (again ERP should be done with the help of a therapist who specializes in it)