Settlement visa for my son

HI All,

I am holding British passport, and now want to bring my son (14) to join me in the UK.
He has no mother( passed away in 2008) and lives with uncle and aunty, but his mother and I were not married and our relationship was broken in year 2005, after my son was born (2004). But her family embers got a death certificate with my name as husband( even we never got married) this is completely without my knowledge and I only came to know very recently, therefore my sons passport has his mother name with my surname.
After our relationship was broken, I got married in 2006( before she died) with my current wife and we both came to UK.

Now my question is when I apply for a my son's visa, I think I need to submit my son's mother's death certificate, when immigration officer see this, will he/she need any further proofs that we both were not married, it was just a relation ship? if so what would be the best thing to prove this?
what would be your guess in this scenario , I would appreciate your time to answer this.

Many thanks in advance.