I'm so sorry to hear of your loss Pesto. There's something surreal about having to do all the practical stuff at a time like this, isn't there? You should be able to find his NI number on a payslip or P60, or his NHS number on a hospital appointment letter or your GP surgery. Take care x
Call your doctors as they will have his NHS number and details of how to get a card.NI number is often on payslips and pensions statements and p60's. his work may give it to you if you can't find any of that.
Good luck with organising it all and keeping it together.