ESA contribution only and total confusion

My husband is aged 62 and has always worked. Last November following a recurrence of depression and being told he needed a knee replacement ( plus high blood pressure and sleep apnoea ) he went off sick. He claimed Esa - not entitled to any monetary Esa as he has a private pension but he was told to go for the WCA for his NI credits to be covered, long story, they got his address mixed up on the system ( official complaint made as it got very messy ) so he didn't attend the WCA as he never received the letters, anyway he eventually had a WCA appointment and was found fit for work, totally disillusioned, he never appealed as he had by now been told he didn't need NI credits anyway as he had 43 years in the pot, sorry bear with me!!
But what's happened now is this
- he has now had conflicting info about the need for NI credits so is unsure whether he should still be paying or not. We've been told once yu have 30 years you dnt need anymore but I'm not so sure as she was contracted out so I think the rules are different but no one seems to know!

- most importantly he went to see GP ( locum ) for renewal of fit note and the fact he had been found fit for work was noted and he was reluctantly given a fit note, he needs the fit note because we have a payment protection policy that pays out because he is sick.
Who makes the ultimate decision on fit notes - the GP or DWP. Had he never claimed ESA none of this would have happened !
They don't make it easy do they - does anyone know the answer to my query?

Thanks Gordon, yes we have been down the pension forecast route and even spoken to the pension service, they confirmed he has 43 years but as he contracted out when in the military it was 43 years in a reduced NI rate so may not be worth 43 years or even 35. So confusing,

Yiu say a fit note is a medical decision. But if someone has been found fit for work, can the GP overrule that and continue with the issuing of fit notes? They won't be submitted to Dwp for reasons explained, they are just needed for the payment protection policy we have?