No secret information is required for Harry to come to his conclusion of “So you really are his one weak point. Huh.”
Available evidence:
Lucius is well known as a hard bastard (initially supplied by random customer, reinforced by Draco)
Draco is well cared for (he is healthy, very well dressed, displays no social anxieties, worships his father)
Draco is probably indulged or even spoiled (Draco’s behaviour)
Draco is being groomed to be Lucius’ successor and therefore his equal (very clear by this point)
From this evidence it can be reasonably concluded that Lucius loves his son. For a hard man like Lucius, this makes Draco his likely weakest point.
Lucius is simply underestimating Harry’s ability to make good use of the available information (and possibly also underestimating how much Draco has given away while trying to cultivate Harry).
EDIT: 25 Jan 2012 - I just noticed that a previous incomplete revision of this comment appeared below by accident. It is now retracted...
Nice. I like your explanation much better than mine. Keeping in mind that Lucius knows very little at this point about what Harry is like — and that Harry is only eleven! — I guess it’s reasonable for Lucius to assume that such an observation by a fellow student of Draco’s would require some adult tutelage.
I just realised that I missed another very strong piece of evidence which immediately precedes Harry’s statement: Draco states that Harry should meet Lucius—he is actually offering Harry privileged access to his father.
This strengthens my view that Harry has noted that Draco offers (multiple paths to) influence with or threat to Lucius.
No secret information is required for Harry to come to his conclusion of “So you really are his one weak point. Huh.”
Available evidence:
Lucius is well known as a hard bastard (initially supplied by random customer, reinforced by Draco)
Draco is well cared for (he is healthy, very well dressed, displays no social anxieties, worships his father)
Draco is probably indulged or even spoiled (Draco’s behaviour)
Draco is being groomed to be Lucius’ successor and therefore his equal (very clear by this point)
From this evidence it can be reasonably concluded that Lucius loves his son.
For a hard man like Lucius, this makes Draco his likely weakest point.
Lucius is simply underestimating Harry’s ability to make good use of the available information (and possibly also underestimating how much Draco has given away while trying to cultivate Harry).
EDIT: 25 Jan 2012 - I just noticed that a previous incomplete revision of this comment appeared below by accident. It is now retracted...
Nice. I like your explanation much better than mine. Keeping in mind that Lucius knows very little at this point about what Harry is like — and that Harry is only eleven! — I guess it’s reasonable for Lucius to assume that such an observation by a fellow student of Draco’s would require some adult tutelage.
I just realised that I missed another very strong piece of evidence which immediately precedes Harry’s statement: Draco states that Harry should meet Lucius—he is actually offering Harry privileged access to his father.
This strengthens my view that Harry has noted that Draco offers (multiple paths to) influence with or threat to Lucius.