Please advise

My son will graduate this year with a bachelor's degree in Biomedical Engineering. He applied some top PHD programs in the same field but failed (haven't heard anything yet). So he is planing to look for a job and also apply some Master's program now (if it is not too late). If apply Master's program, he is thinking of majors other than Biomedical Engineering. He likes Math and programming. Is Applied Math a good major or Computer Since or Bio-Statistics ? Can some friends here advise?

Thanks a lot in advance.