why not "it" here
You're right, the sentence would make sense as well if it was about a box, so "it" should work. Added.
Are you saying nim can be translated as it?
Why "Say me something about him" is not correct?
That is grammatically wrong. You can say "Tell me something" or "Say something to me". The second option sounds rather strange in this particular example.
"Say something about him to me" would sound fine enough in English, but I see that it is not accepted. Is there another nuance to Polish that would make this an incorrect translation?
Sounds fine, added.
These two words/phrase, powiedzmy and powiedz mi sound the same to me. Is it only me?
Well, a native should hear a difference, provided that the person speaking wasn't mumbling, but to a learner... I'm not surprised that they're similar.
They sound quite different to my ears, there's rather a similarity between y and e. But that may depend on one's experiences with other languages.