Harry saying that Draco was Lucius' only weak point was probably just an (accurate) surmise given the available evidence: * 1. Lucius is well known as a hard bastard (information from random customer) * 2. Draco is visibly well cared for (and his behaviour suggests that he is indulged or even spoiled in many ways) * 3. Draco is being raised to be his father's successor and equal

Therefore Lucius probably loves Draco, making him a weak point.

No secret information is actually required to make this assessment, though it might be considered a bit close to a guess.