This is an employment/contract law issue. In simple terms, an employer cannot change the terms and conditions of an employees contract without their consent - contracts are at the very least, bilateral agreements. Change without consent is a breach of contract.
But realistically employers can pretty much do what they like since the employee must (after following a grievance process) resign and then claim unfair dismissal. Even then, for the claim that the contract has been breached to be successful, the breach must be 'material'.
The breach (supposing it occurs after your son has exhausted the grievance process) could well be material, because it will affect his salary/income. The question is: does your son lose out by taking on the H.P.?
H.P. is quite expensive, but on the other hand, the expense of enetering an H.P. agreement may be cancelled out by benefiting from paying less income tax as a company car is a taxable benefit.
Rather than taking legal advice, I think at this stage your son should seek the advice of an accountant. If he loses out from this change then he should raise a grievance, but he may actually gain from the proposed change.