koseki tohon entry correction

My son was acknowledge by his japanese father & registered in his koseki tohon after we got married. The problem is,we just noticed that the birth year of my son's birth certificate in the city hall (philippines) & in NSO is different. In LCR manila which is correct is year 2000 but in NSO is 1999.There is typographical error so to make it corrected we file a petition by court & now it is all done. So no,we have a new copy of birth certificate of my son from NSO year 2000.

The birth certificate (nso copy) we submitted for my sons acknowledgement & registration in koseki tohon is year 1999. How we will make a correction to change that birth year in koseki tohon from 1999 to 2000? Please help us.thank u

You and your son need to take a legal action in the JP family court to correct the birth year.
Once it has accepted, the local city office where you son' koseki is kept will renew the birth year.
The court process will take time and money.

Do we need to hire a lawyer or we can do it on our own personally? My husband is going to family court for this matter but do I need & my son need to come with him to the family court?

If you or your husband live in Japan, it is either lawyer or judicial scrivener (Shiho-shosi) can do it.
Your local city office normally provides a free legal consultation. So I would recommend you take a free legal consultation first.

Thank you very for helping me & enlighten me regarding this matter.I will do all your advices it's really give me a big relief.

Another thing, once everything is done & corrected. Because my son is in his father's koseki tohon as acknowledge,does it mean that he Japanese national already or we have to do something to acquire his father's nationality so he can get a Japanese passport.Because for now,he got Philippine passport only.Thank u

While your son is registered under Koseki, your son has a Japanese citizenship, so it's possible to apply for a JP passport. He can hold both JP and Philippine citizenship and passport until he turns 22 yrs old.
Foreigners(ie, non JP citizens) are unable to set Koseki, therefore, your son must registered under JP father's koseki.