Can a bailiff clamp my car for my son's debt?

No

Bailiffs may only take control of vehicles that belong to the debtor.^[1]

If a bailiff clamps someone else's car, the owner can make a third-party claim.^[2][3]

The owner of the car may also recover damages,^[4] together with costs for bringing the claim.^[5][6][7]

^{[1] Paragraph 10 of Schedule 12 of the Tribunals, Courts and Enforcement Act 2007
[2] Civil Procedure Rule 85.4
[3] See Making a third-party claim to controlled goods
[4] Section 3 of the Torts (Interference with Goods) Act 1977
[5] Civil Procedure Rule 46.5
[6] Civil Procedure Rule 46.5
[7] Practice Direction 46.5 Rule 3.1}