OK, I'll be the bad dad. I would and have done exactly that. A bike is quite stable at high speed because of centrifugal force acting on the wheels, and shifting your weight exerts as much control as the front wheel does.
You have already committed the deed just by letting the child ride at all, the extra danger of letting him take the bars is minimal. You are right there after all.
The greatest danger is when mom finds out afterwards! Never film these shared moments of machismo, It's difficult to minimize the event when mommy is looking right at it!