Games : My son is a moron and I don't want him inheriting my empire. Why can't I nominate him as a bishop?

This is my firstborn son, Enrique. He's a total moron. Unfortunately, he's also my heir. (I have primogeniture succession and papal investiture.)

My usual plan would be to nominate him for a Bishopric via the Pope. The Pope adores me (92 opinion). Sadly, Enrique doesn't show up under the list of people I can nominate. He's definitely at my court, and he doesn't own any land.

Why can't I nominate my son for a Bishopric?