Well, I agree that Plutarch the one that mentions that Brutus thought he might have been Caesar's son is a long time after. Suetonius's uses and you too child. But I agree Caesar was kind of young for an affair with a marry woman at 16. It does happen sometimes but I doubt that Brutus was his son. I don't know why he was fond of him. Maybe it was because of Servilia being a ex-mistress and friend that he developed a fondness for Brutus.