He has no choice but to travel in taxis in which there is haraam music

You have to advise those with whom you travel in taxis in which there is haraam music not to tune the radio in to music stations, in the hope that they will respond to your advice and refrain. If they heed your advice then praise be to Allaah, otherwise then you should get out and leave them to their misguided ways, so as to protect yourself from listening to evil. If it is too difficult for you to get out for some reason, then there is no sin on you if you stay in the car. 

And Allaah is the Source of strength. May Allaah send blessings and peace upon our Prophet Muhammad and his family and companions. End quote. 

Standing Committee for Academic Research and Issuing Fatwas.