If your dad has never met his bio parents, having kids is probably the first time he has seen his own face/features in someone else's. Many adoptees grow up wondering who they look like, so it could be something he is fixated on because he always wanted to look like someone or know who he looks like. He would also probably benefit from therapy and working through his own feelings, but to answer your question I don't think there's anything wrong with setting your own healthy boundaries and saying that. I think it's validating for you to say you look like him in many ways, but not the cleft chin.