정답은 him 앞에 to 가 붙어야 하기 때문입니다~!
each other, him, her 이 목적격으로 쓰일때 대명사이므로 to 가 붙는게 맞습니다.

I was talking him (X)
I was talking to him(O)

나는 그와 말하고 있었다.

They were talking each other for 30 minutes (X)
They were talking to each other for 30 minutes (O)

그들은 30분 동안 서로 얘기했다.

I was able to communicate him(X)
I was able to communicate with him (O)

나는 그와 대화할 수 있었다.

to 없이 each other 을 쓸 수 있는 경우는?

예문) How long have you know each other?
두사람 알게된지 얼마나 됐어요?