OCI for a Minor : Letter contents?

I'm applying for my Son's OCI (I already have an OCI).

I keep getting asked for additional documents with each submission, the latest being a letter "as my son was born before I was married".

Does anyone know what needs to be included in this letter? Do I just state that my son was born on date X with partner, and we didn't decide to get married until a few years later? (lived in a country where getting married means higher taxes)