He doesn't, he, like me, has missing years that he is considering purchasing to increase his pension when he reaches state retirement age.
However, he will not be able to buy any missing years prior to (I believe) 2016, I stand to be corrected on that.
To find out what is available you have to contact the Future Pensions department directly or use your Government Gateway account.
After that you can either send a cheque in the post if you have a UK bank account or if you wish to pay electronically you require an 18 digit reference number which can be obtained by calling HMRC.
There are many topics about these issues on this forum https://forums.moneysavingexpert.com/categories/pensions-annuities-retirement-planning