If you recently go married, how can you change your child's surname to his father's?

What we did was go to civil registry and ask for requirements. From experience, requirements varies. A friend of mine were required that both parents be present during filing, for an interview. But with mine, we completed the given requirements but only a few were considered. There were also new requirements added to the list, It was a good thing we called before traveling from manila-cebu. Took only a day to process and got the result in less than 2 months.

This might help! https://ph.theasianparent.com/how-to-legitimize-an-illegitimate-child-after-marriage/