J.K.
"The talk" is really an ongoing series of discussions to start around age 4-5 with body parts and privacy, body autonomy and safety. It continues on over the years with more detailed information being given as they grow older.
It can be awkward but he wouldn't have mentioned it it hasn't been weighing on his mind.
An 8 year old doesn't need gritty details or graphic anatomical charts, but he should know the simple basics of where babies come from. Start with finding out specifically what he overheard, because you need to know that to confirm/refute. From there you will know what kind of explanation he needs at the moment. Go simple, let him think, wait for the next question before providing more detail.