Getting a pio card

I would very much appreciate if somebody would advise me on how to go about sort my problem getting the pio card. I am a resident of Uk and retired. My Father was born in Kollam,Kerala 0n 1920 according his canadian passport.He passed away last year.Through my relation in Kerala I can only get a nativity certificate instead of a birth certificate.I suppose his birth was never registered in that period of time.Will this nativity cerificate sbstitute the birth cerificate for the pio card,if not,is there any other way? Currently I have his nativiy cerificate,his old canadian passport and death cerificate stating his birth place Kerala issued in Canada.He has no school cerificate evidence as he left India.only evidence his relation are still there and they have no clue about his education etc.
Thank you for you time

Not an expert, and certainly not a professional, but --- it sounds good so far!

Do you have any school certificate or document of his from India? That might be useful.

All you can do, having gathered up all the evidence you can find or obtain, is go for it and apply!

Thanks.Having seen other nationals questions,i am also a srilankan origin( does it mean i will have un upheval task for the PIO?) settled in Uk for the last 36 years. But I am a frequent visitor to my fathers native place. As I mentioned above I have no proof other than the his close relations still there. can any of the relation do anything to proof of my fathers birth?
thanks

Mention of Sri Lanka might make things harder. From the Immigration Bureau website