"Do not tell him about it."
Translation:Nie mów mu o tym.
Not here. If you wanted your interlocutor to actually do tell that, then you would need "powiedz" - because that would be a one-time action, perfective.
But as not telling is... not telling ever, you need imperfective "nie mów".
Yes, the meaning is the same but you cannot use them interchangeably - it depends on the sentence/word order (like "mi - mnie" or "cię - ciebie").
No, unless you have some context, in which you transform this sentence in order to stress the pronoun. For example:
I told his mother about it. Don't tell him (about it). = Powiedziałem o tym jego matce. Jemu o tym nie mów.