If you're on the event step where he is inside of your castle, then you can find him on the left side of the sleeping chambers, below the room in which Sandra is.
In that case, he can be found in the bar in Aldlyn. But that's where his story ends when you decide to return him to male.
You need the "Monster Craft Recipe" to craft those things, which you get from Vivian after the events of capturing monsters allowed you to build a monsterhall in your castle.
That comment by Trey is about Mirel's event. They continue in her events, not in Trey's story.