He may also have been ignorant of how much danger he was in. ATC will be happier with that answer than having to roll the crash trucks. You may be asked to circle or hold until traffic thins out, but that is a fair trade. Similarly, there may be traffic on short final when the tower clears you for immediate takeoff. If you're ready, fine.
Wake turbulence is a well-defined hazard.
Big airplanes churn and curl the air, making it impassable for small aircraft behind and below them. When flying from predominantly air carrier airports, light-aircraft pilots must be especially vigilant. The tower typically is used to heavy iron. Big airplanes don't usually pose a grave threat to other big airplanes, although there are a few exceptions. On several occasions, I've been cleared for takeoff immediately after a large aircraft and have requested an immediate turn to avoid the wake.