It is the law that he has to show he is legally entitled to work in Britain, if you look at the staff handbook it will list the combinations of forms of ID that are acceptable. usually it is a passport, but other combinations can be used.
the single document:
A passport showing that the holder is a British citizen or a citizen of the United Kingdom and Colonies having the right of abode in the United Kingdom.
Combination documentation:
P45, P60, NI number card or letter from an approved Government agency. (Please note that a National Insurance number does not automatically indicate that the individual is eligible to work, and is not sufficient evidence), plus:
Full UK, Channel Islands, Isle of Man or Irish birth certificate, or
Full UK, Channel Islands, Isle of Man or Irish adoption certificate, or