No, that would be "Ich weiß nicht über ihn." There is a difference between nichts and nicht. Nichts meaning nothing, nicht negating the verb.
"I know nothing about him" was accepted for me, so I guess they fixed it? :)
nichts means nothing at all, so it's : i don't know anything about him . If it were 'nicht' , it would be ' i don't know about him'
I'd actually visited this link before I posted, but it mentions "motion versus location", and when there's motion to a location, then a "two-way preposition" (like über) should take accusative, if no motion, then it should take dative. So...in this example I don't see location or motion, so.....it is still unclear to me.
I can see why it is unclear --no motion nor location in this example. And I'm sorry--I may have misunderstood your question, too, since ihn is used in the accusative. Sorry I'm not much help :)
Well, after a little research, apparently when "über" means "about", it should be followed by accusative. Thanks for your help!
I'm a little frustrated with the difficulty in differentiating between ihm and ihn in the spoken question. :(