Your dad is right. On desktop, navigation is on the left. On tablet, you shrink it to a rail. On mobile it should be a dismissible nav drawer.
The top menus, especially the flyover(on mouse hover), are bad for accessibility because they convert a non-committal action (hover) to a context changing one (focus). It’s a uniquely web-only invention and thankfully falling out of usage. (Unless you mean menubar/toolbar. Those are fine but extremely rare on Web.)