Revoked unconditional offer

My son applied to university through UCAS and got a conditional offer subject to achieving the required A level grades. Two weeks before results day, the university sent him an unconditional offer. A day later, the university sent him an email stating that the unconditional offer was sent due to a system error and was no longer valid. Can the university withdraw the unconditional offer? Is the unconditional offer not a legally biding document? Should my son request the unconditional offer to be honored?

If it was sent in error then they can revoke it
Does your son think he won’t meet the conditions?