You can always talk about big, important things without focusing on your relationship (you know, God, politics, the environment, your dreams, etc). That gets both of you used to a more meaningful conversation without scaring him.
Of course, if that doesn't work, there are lots of fabulous books of questions on the market. Even if you just play the game where you randomly open the book to a question, chances are it'll be a question you want to know the answer to!
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