Sorry, I couldn't type the accents so now I copied. The personal pronoun chart has ně and něj in the same box on the ono line, Acc. after prep, as well as ně in the oni, ony, ona (pl.) line. So ně can be singular or plural, yes? So aren't ně and něj interchangeable except for when you mean plural?
Oh, you mean neuter. OK, I was quite confused because normally one would understand this sentence as masculine unless a neuter subject is clear from the context.
Yeh, for neuter singular accusative there are many forms possible indeed. You can also check http://prirucka.ujc.cas.cz/?id=ono
Both "o ně" and "o něj" are possible neuter singular but I feel that "pro ně" is really bookish and I would never say that and I would hesitate about writing it actively.
As the links shown by va-diim show, it is masculine, "for him". You are probably reading the textbook wrong, they shouldn't really say what you seem to read there.
This word is confusing for other Slavic languages' speakers because there is "о неё" (or "про неё" /pro nějo/ and that is indeed "about her").