Hi,
I presume that you mean National Insurance Stamps?
He would be best to contact the Inland Revenue, they would be able to give him specific information for his case, however, i think that it is probably unlikely that he would be able to do this if he is already retired. I know that you can buy extra before retiral, but I understand that this can be quite expensive.
If he doesn't have any other income, he should maybe approach the benefits agency, as I think there is some sort of minimum income he should have and he may be entitled to some benefits to supplement his pension.