My boyfriend always doubts that he finds himself visually appealing. How can I reassure him he is appealing?

My boyfriend, a boy I'll call Ilia*, always doubts that I find him visually appealing. His sister, a girl I'll call Tatiyana*, keeps telling him about how I think otherwise and he doesn't believe her. I always tell him I find him very good-looking and better looking than Brad Pitt. He tells me not to doubt himself and he does it in front of me. It makes me self-conscious when he gets self-conscious. How can I reassure Ilia* that I find him good-looking and I could care less about looks?