because they are loyal, full of gratitude, shock-proof (once the initial 2 secs revelation is over), and constantly guilt-ridden.
Reveal their true identity only when they choose to marry the girl you don't approve of. Whoever told you adoption was a risky bargain for wealthy, aristocratic, stiff upper-lipped, "desi" families like yours was totally insane. Afterall, you have given him love, food and shelter. Drive your son to the verge of abusing prescription drugs by admonishing him how his ultra-english upbringing was not good enough to turn his blood to the perfect shade of blue you wanted. Hereditary.. schmereditary. He will come around... because he is, after all, the most grateful and loyal of all creatures.