Praise be to Allaah.
He does not have to pray Asr a second time, because when he combined Asr with Zuhr, he was in a state when doing so is permitted, and he did not know for sure when he combined the two prayers that he would reach his destination before Asr. Even if he did know for sure when he combined Asr with Zuhr that he would reach his destination before Asr, it is still permissible for him to join the two prayers, as was stated in a fatwa by many of the scholars.
This scenario resembles another, which is when a man cannot find water, so he prays with tayammum (dry ablution performed with sand or earth), then he finds water before the time for that prayer is over: he does not have to repeat the prayer.
In the case of the traveller, the time of Zuhr is in fact the time of Zuhr and Asr, and the time of Asr is the time of Asr and Zuhr. And Allaah knows best.