Dementia can cause a number delusions, illusions, obsessions even hallucinations. I empathize with you as you navigate handling this with your husband. When my dad’s became to the point that he was distressed or agitated I discussed it with his doctor and he prescribed medication that really helped. I found that trying to convince someone who has these illusions or delusions that they are mistaken is useless. I often said we’d address it later. I’d keep saying that until the obsession faded. The medication helped calm their thinking and manage anxiety.