Could the Son be an indicator of the Gods' intervention? They can't see the No God, but they can see Khellus. It seems the Judging Eye opens when necessary, not when asked, which means it probably has some connection to the Gods. If the boy was meant to survive, it explains the Survivor's un-Dunyain behavior to him, and eventual salvation (?) at the end of his life. So could the Gods have chosen the Son to be their agent? Of what...I don't know know, maybe the next book.
Also, unless every Dunyain was named Anasarimbor, which is a real possibility, what logical evidence does the Surivior know it was his son? Seems to be a guess based on feeling and fatherly instinct, something lacking in the Dunyain.
Was there is a reason why Moeghus called his son (was there any way to know Khellus was his, outside of Cnaiur's memory, but the only other Dunyain adult we have met alive was horribly scarred, they could be clones for all we know) or did his simply call the most promising and talented of that generation?