You're right, the sentence would make sense as well if it was about a box, so "it" should work. Added.
Sure, „nim” is for masculine nouns just as „niej” is for feminine nouns, irrespective of the sex/gender of the things you refer to:
- –Hoduję pająka(m.) w domu.
- –Interesujące… Powiedz mi coś o nim.
–To samica Czarnej Wdowy.
–I'm rearing a spider in my house.
- –Interesting… Tell me something about it.
–It's a female Black Widow.
–Hej, przykro mi że rozbiłem ci tę butelkę(f.), po prostu jej nie zauważyłem.
- –Hey, I'm sorry I shattered this bottle of yours, I just haven't noticed it.
And so on and so forth – this frequently causes problems in English for Polish speakers course, as Polish people have serious problems with understanding why they should use 'its' when referring to cat's milk in sub-clause, for example. ;)
Just as 'wiedzieć' doesn't turn into imperative 'wiedź'.
Also, 'powiedź' and 'wiedź' are actually imperative of 'powieść' i 'wieść' = imperfective and perfective verbs which can be translated as 'to lead'.
Why "Say me something about him" is not correct?