Kogoro isn't lazy when the situation calls for it, he's actually a frighteningly effective detective in dire situations. However, I'm very sure that Conan's identity isn't the first thing on his mind, so he just brushes it off as a kid being smart-aleck.
Besides, to him, Shinichi and Conan aren't linked in the slightest, so he wouldn't think "Hmmm, maybe this kid is actually that Shinichi brat". The mere fact that Shinichi somehow shrank into a 7-year old Conan defies any standard logic.