Learn Spanish (Spain): Deja el coche donde quepa, hijo. - Leave the car wherever it fits, son.

We use the subjunctive because the exact location is unknown or hasn't been found yet. The speaker is saying to leave the car wherever it happens to fit. If they had a specific, known parking spot in mind that they knew it fit into, they would use the indicative and say donde cabe.

Quepa looks like an unusual word. What verb does it come from?

It comes from the highly irregular verb caber (to fit). Remember that to form the present subjunctive, we take the irregular yo form of the present indicative, which is quepo, and add the opposite verb endings. This gives us the subjunctive forms quepa, quepas, quepa, and so on.

If the sentence started with a negative command like "Don't leave the car", would the rest of the sentence change?

The donde quepa part would stay exactly the same, since the location is still an unknown projection into the future. However, the first verb would change to its negative imperative form, which also uses the subjunctive: No dejes el coche donde quepa.

Is the speaker definitely talking to their literal son by saying hijo?

Not necessarily! In Spain, hijo (and hija) is frequently used as a colloquial term of endearment, familiarity, or mild exasperation. It can be used with actual children, but also with friends, partners, or younger people in general, somewhat like saying "man" or "my boy" in English.