Your brother will not need to apply for another National Insurance number if he had already been allocated a number before he left the UK.
To find out his National Insurance number he has two options. He can either speak to an advisor at HMRC, and if he passes our security checks, and his National Insurance number can be traced it will then be sent to him via post. He must ring HMRC himself.
The contact number for calls from abroad is, +44 191 203 7010 and for calls from the UK it is 0300 200 3500.
Alternatively, he can complete form CA5403 which can be found here:
Get your National Insurance number - GOV.UK (www.gov.uk)