Parental responsibility/guardianship

[Martiens]

I'm a SA citizen with my partner in the UK. We each have a son (6&4) from a previous marriage. My partner is a UK citizen and I have full custody over my son (ex has reasonable access and lives in SA). My partner and I are in a complicated process of getting our partnership registered under UK Law as a 'marriage', which we are told might still take a year or two. I feel the need to give my partner legal parental responsibility/guardianship to make emergency decisions concerning the welfare of my son. My ex agrees to this and will be sending us a letter to the effect. How, what, where and when? What is the legal term that will be applicable to my partners' position concerning my son? Thanks