how to register our baby in his kosekitohon

Im a filipino and my boyfriend is japanese..
me and my bf not yet merried..and now im 9months pregnant. how to registerd our baby in birthcertificate of his father since we are not yet merried.

why you and your Japanese bf dont go to the city hall and ask?

I think a foreigner can't have an advice from city office (or embassies) for baby registration to koseki of a Japanese man, because they don't be married.
some legal (but irregular) process has to be done by a Japanese man.

why don't you get married right now ?

When my child was born I was asking to much to the city hall. And the staff helped me out well.
But I was married before my child was born, so maybe you are right.

For a child born between a non-Japanese mother and a Japanese father to obtain Japanese nationality, the father needs to officially acknowledge the baby-to-be as his, BEFORE BIRTH. The process is called "taiji ninchi" (fetal recognition). If you want the child to obtain Japanese nationality, please be sure to do this promptly.

I phrased it wrong. sorry. It should say:

For a child born between a non-Japanese mother and a Japanese father WHO ARE NOT MARRIED to obtain Japanese nationality,...

What is your purpose?
Do you live now and hope to give birth to the baby in Japan?

In the situation
where you and your boyfriend
will not or cannot get legally married for a certain reason,
your boyfriend and your baby
can become legal father and child
by acknowledgment before or after birth,
then the baby can obtain Japanese nationality.

These two things
are not directly related to in which family register [koseki] the baby is entered.

"To(u)hon" means transcript, i.e. fully copy.
Koseki-tohon is a transcript [tohon] of a family register [koseki].

AK:
the father needs to officially acknowledge the baby-to-be as his, BEFORE BIRTH.

Your baby can obtain Japanese nationality
when he is acknowledged after birth by a Japanese man.
(@AK: Source in Japanese: http://www.moj.go.jp/MINJI/minji163.html)

I recommend before-birth [fetal] acknowledgment (by your boyfriend),
because it enables your baby to obtain Japanese nationality by birth;
at the time of the baby's birth,
your Japanese boyfriend becomes a legal father
by the before-birth acknowledgment.

In a case
where your baby is born
while he has not got acknowledged and you are not legally married,
he does bot obtain Japanese nationality by birth.

Anyway, you had better consult a specialist.
As your first step, if you are in Japan,
I recommended you ask for professional advices
at Japan Legal Support Center,
which is known better by its Japanese nickname Houterasu.
https://www.houterasu.or.jp/en/index.html

One thing to note:
if your baby is born outside Japan
and he obtains a foreign nationality by birth,
unless he indicates an intention to reserve Japanese nationality
when his birth is legally notified,
he should lose Japanese nationality.