Toaru Jihanki no Fanfare mentions Kihara Amata's Son specifically in reference to the Level 6 Shift Project involving the clones created by Amai Ao, therefore it is likely it references Accelerator, as Kihara Amata was in charge of his power development, rather than an biological son.
Accel's surname is stated to have three kanji and "Kihara" has only two, so he is not a Kihara. And it's unlikely that the scientists and the Kiharas would refer to Accel as Amata's son, unless it was a scornful label; they would rather call him Amata's experiment/pet/toy or something like that.
Ops, my bad, I remembered wrong. Anyway, the novel states that his name is a very ordinary one and considering how famous, or infamous, the Kiharas are it's difficult to consider it ordinary. And, as said before, it'd be strange if researchers that consider Accel a specimen talked about him with that epithet.
I agree with Kamina it can't be accel as he wasn't involved in the dark side of AC till the level 6 shift and its stated by Nayuta he's been involved in the darkside for sometime as for Blue hair we can scratch him off due to his fight with Touma at NT10 he didn't display any abilities or knowlegde during the battle since they havent introduce any Kihara who's ordinairy and useless and considering Kagun's statement he hints that all if not most Kihara's are involved or formetly involved in the darkside of AC
Aogami wouldn't show his abilities also Kihara is ordinary the members aren't and maybe either Accel or Aogami are the son but one things clear, they might and i say might be teenage kiharas. especially Accelerator.