Should the son take the dad literally by moving forwards and then backwards - in which case the combined command could have been a contracted version of "Go ahead, and then backup." Or should the son appeal to his years of knowledge of the English language, filtering the expression and just backup up the car. The brain would have then interpreted the command "Go ahead backup." as "Now do this: Backup."
Usually such a double meaning riddle are asked of little tots to test their comprehension of English - well maybe not to test it, but just to watch their confusion in sorting out the "true meaning" within the context of the situation, and more confusion stemming from a ( deliberate ) convoluted explanation.
Note that "Go ahead backup." can also be interpreted as two separate commands with a suitable pause between, ie Go Ahead. Backup, in which case it becomes a recipe for the recipient to follow from start to finish.
You may also note that lawyers receive vast sums of money trying to sort out riddles such as this when plaintiff and defendant square off., or in some other court proceedings. Not necessarily the only place where language riddles come into play - workplace, home life, politics, novels,.. The riddle is a simple description of where in the real life drama can result between two people if meaning does not correlate exactly between them, either by design or effort.
It is kind of what was meant, what was said, what was heard, and what was interpreted.
Capice? :)
... and of course Logicians.