When to set up government gateway ID to change name

My son needs to change his name for HMRC & DWP. He does not have a government gateway ID yet. He has changed his name by unenrolled deed poll and just sent off for new drivers license and US Passport. He has to wait to have US passport returned in new name before sending off for new British passport. Should he set up government gateway ID in old name now while he still has his old passport then change name after new British passport arrives or should he wait until he receives British passport with new name?

Hi 

Your son should contact us by webchat or phone via Income Tax: general enquiries
to confirm the update of his new name on his PAYE record.
He should then wait to receive his new passport, before setting up Government Gateway ID using this passport as confirmation of his new name.

Thank you