How can I get help for my father-in-law who is in complete denial about his dementia?

My father-in-law, who lives in our guest house, has been showing obvious signs of paranoid dementia for the past several years. It’s becoming quite a problem now, as he got combative with my husband and has yelled at people in public and has become convinced that everyone is trying to rob him. He accuses us of the most random, silly things like stealing some of his spices (not the whole bottle, just taking some out of the bottle), replacing his spaghetti sauce with different sauce, switching his flashlights, etc. He becomes very angry and confrontational about these things, and it’s putting a huge strain on our relationship with him. Especially my husband (his son and only child). He is completely convinced that he is mentally sound and that we are stealing from him and refuses to seek help. We managed to get into a therapist with him, but once she mentioned that she felt like he was having delusions and should get a brain scan, he won’t go back. How do I get him help since he is convinced there isn’t an issue and that his delusions are real? I just don’t know what to do anymore. The therapist helped give us tools on how to deal with him, but I don’t know if I can handle the constant fighting and accusations without getting defensive myself. Help please!