Let's start with a civil analogy. If you owe someone money, and youdon't pay up, they can apply to a court for a judgment ordering you to pay. If you still don't pay up, the person you owe money to can then ask the court to order your employer to make payments to them directly from your wages. Anyone can do that; it's not just 'the authorities' that can. It's a common way of ensuring that debts get paid.
Your son has incurred a debt but there's no need for 'the authorities' to seek a court judgment against him, as (through issuing a fine) a court has already ordered him to pay the money. So enforcement action can now be used against him, such as having the money taken directly from his wages.
Your son is lucky that an alternative way of getting the money hasn't been sought. If the decision to use court bailiffs had been taken, it would have cost him far more. For example, if they took a telly that cost him £500, potential purchasers at auction couldn't be sure that it was fully working, so it might only sell for £80. After the auctioneer's fees and bailiff's fees had come out of that, there might be no more than £50 left to go towards paying off the debt. (i.e. it can cost TEN times as much to pay back debt through bailiffs than it can through having the money taking from one's wages).
He's also lucky not to have been prosecuted for failing to notify DVLA of his change of address, which is a criminal offence.