How do you get a National Insurance number if you’ve lived in the states most of your life?

I wasn’t sure which flair this fit under or if this is the right sub…

My husband (35) has dual citizenship, he was born in the UK. He has lived in the states since he was 6 months old. We are wanting to move to the UK to be closer to his family. The jobs he has been interested in require the NI number for the application but he doesn’t have one. How can he get one?

EDIT: if we are currently living in the states, how can my husband apply for jobs if he doesn’t have an NI number? Can he travel to the UK, apply for the number and do the interview, then travel back to the states and continue his applications? I can’t afford our monthly bills by myself if the only option is for him to move there first for 3-6 months then come back to get me.

Has anyone been in this situation that can assist?