YOU SAID:

and that's when he knew he had pooped his pants

INTO JAPANESE

その時、彼は自分のズボンにうんちをしたことを知った

BACK INTO ENGLISH

That's when he knew he pooped in his pants

INTO JAPANESE

その時、彼は自分がズボンの中にうんちをしたことを知った

BACK INTO ENGLISH

That's when he knew he pooped in his pants