He can drive either while on his American license. Bearing in mind that he need to follow all UK driving laws and be able to drive a manual car safely.
I would start the process right away of getting his UK license because if he hasn't done it within a year (ie, finished and passed everything) he won't be able to drive.
He will effectively start from scratch with a UK license. I would HIGHLY recommend lessons as they pretty much are trying to teach you to pass the test (oh and drive, I guess). So he has to pass the theory and practical tests within a year. He can choose to just do an automatic license (I did), but will only then be allowed to drive automatic cars.
I did know a guy where it took DVLA like 4 months to finally send him his provisional license, so this can take time too!