Castiel most certainly would not tell Dean about whether or not Sam was eating or not: the angel hardly considering things like food being a thing that were practical or necessary. He was no one's babysitter, after all.
Still, he sipped his coffee and considered. "Just tell him what's necessary," he said after a moment. Soon enough he'd know that angels existed anyway. It wasn't as if Castiel could hide what he was, nor would he want to even if he could.
"And we could research… not this place. But perhaps this phenomenon? Perhaps there is some lore about it."