This man has definitely spent many a coffee-fueled night tinkering with rune spells and theories while completely losing track of how many hours he's been at it. He's often surrounded by several half-drunk mugs, each abandoned because he forgot about them and the coffee went cold.
That said, warm Earl Grey tea with milk and sugar would be a nostalgic comfort drink for him as well. So both, depending on whether he's feeling calm or whether he's hyperfocused on something.
Also whisky, but that wasn't part of the question. And he definitely puts whisky in his coffee.