Is that slight brain disorder possibly autism, specifically Asperger's disorder? People with Asperger's often tell the literal truth without an awareness of it's impact on the other person, whereas most of us would say "Now, you know there's no one prettier than you."
If that is the case, I think the best thing to do would be to just let the remark about the other girls being prettier slide. If most guys said that, I would say they were sending you a message. People with Asperger's don't imply things. If he has Asperger's, or another condition of the autistic spectrum, then that remark was just a statement of fact and nothing more. (I presume you don't actually think you
are the prettiest girl in the world.
)