How to use father's surname for illegitimate child

The father of my child and I are not yet married but we would both like for our child who is due to be born some time in June to have his surname. He will be in the US for his internship and will be gone from April onward. He will be staying there for approximately 6 months to a year. Is there any way for him to leave an affidavit or something to legalize the use of his surname? Which government office can we talk to on this matter?

Also, it is impossible for him to go to the Embassy of the Philippines in US because he will be staying in a different state. One not accessible to the embassy.

Your options some requiring strong intentions from the father if you want the child to be legitimate or carry the father's name but as an illegitimate:
1. Norm for regular birth registration with the Local Office of Civil Registry is 20 days. You may defer for a late registration when the Affidavit of Paternity/ to Use Surname is available but there are more requisites to comply with. Your partner if he is willing should be able to find the time to have this ribbonized in the US if he wants to. For late registration, the LOCR will initially look for a Negative Certificate of Birth Registration (Certificate of Non Registration).
2. Get married before he leaves to legitimize the child. You and him may have reasons for not doing this.
3. Register with the LOCR within the prescribed time and available Certificate of Live Birth using maternal name as the child's surname. With the proper documentation, file a petition in court to change the child's name to his father's. This requires a lawyer and takes time and effort.

There maybe other workarounds. Leaving an affidavit? Check with the LOCR or also at the hospital where you plan to deliver the baby. Make sure the certificates are signed by the attendant at birth.

Ask ko lang po me pag asa lo ba ma iligitimate nalang ako ora lang mkuha ko last name ng tatay ko since void naman kasal nila dahil kasal mama ko sa una. Ngaun ung sa ausf na nabasa kk affidavit if latternity etc. Ask ko po paano po un saakin nasa local civil registry ko at the back of the page me affidavit if acknowledgement na kask nakalagay my subsequent marriage may date pa nga poe eh maffile ko la dn kaya yan as illigitimte? I badly needed my last name help