If Draco already had an ability to cast a patronus, it may now be another thing harry has taken. This may make him feel obligated to tell the secret.

Given that Hermione already can't cast a patronus, and for similar reasons as Harry I rather hope that he is giving her training too. Starting with necessary background if necessary. In this instance that will more or less the distinguishing factor between 'pretentious short-sighted cleverness" and wise restraint.