YOU SAID:

I am friends with him.

INTO JAPANESE

私は彼と友達です。

BACK INTO ENGLISH

I am a friend of him.

INTO JAPANESE

私は彼の友達です。

BACK INTO ENGLISH

I am his friend

INTO JAPANESE

友人だとも！

BACK INTO ENGLISH

I am his friend!

INTO JAPANESE

友人だとも！

BACK INTO ENGLISH

I am his friend!