paternity acknowledgment

I'm just curious if the birth certificate of my son is acceptable and legal. He was an illegitimate child and the biological father left when I gave birth. I was ready to file his birth certificate without even the name of his father then when my mom's friend suddenly told us "di pwede na walang name ng father, kawawa naman ang bata if he'll see his birth certificate na NAME OF FATHER:UNKNOWN". So he volunteered to sign and acknowledged my son. He also paid for my hospital bill. This kind person is now married and has his own children but still gives my son support (tuition fees) and he knows that he is really not the father. Somebody told me that it is not binding. Out of curiosity, I would like to seek for your legal opinion on the legality of the acknowledgement of paternity. Thank you very much!

unless, it is attacked in court, it is presumed to be valid