I never said there was a need to be married. Just that the child had to be legitimized with parental rights granted to the father in order to use that for an extension.
No. As I said. You must be married, and your name must be on the birth certificate. Both of those must be that way when the child is born. If either is not that way, then you do not get parental rights and must obtain them through the court.
And how old was the child? The information I outlined is for infants. Once the child is older you can just do it at an amphur office. But based on the post, the op cannot wait ~8 years for that