Will he need to get a work permit or visa
nope, just flight/coach ticket. no work permit or visa required.
and will he have to apply for a National Insurance number , before he can start working here?
he can ring the national insurance office (or something like that, check in the yellow pages or google it) and arrange an interview, in the meantime he might request a temporary insurance number to provide for his employer untill he gets a proper one, he will need to tell them his year of birth. he will need his passport for the interview. how's his English? if its not good they will get him a translator present at the interview for the nat. ins. no. he will also need to inform them about his address in uk.
edit: under no circumstainces should he work for someone without a national insurance number. they will most probably tell him on his pay day that they cant pay him cos he hasnt got one and its 'illegal'. a lot of Poles and other immigrants get messed about like this. they are happy for you to work, but wont pay you.