Most importantly, if my character (again, without hormone thereapy) passes well, if he looks like a man to a casual observer, would he be able to sing without outing himself?
I've googled all sorts of combinations of transgender/transexual/transman and voice/singing.
There's a lot of info out there on how a person's voice changes after they start hormone therapy and how this relates to singing, but most of it focuses on pop styled singing and there's very little about classical singing. And I haven't managed to find anything useful about things that can be done with the singing voice (that aren't bad for it, like vocal fry) before hormones. though there's a good deal of advice on how one can speak in order to sound more or less male or female.
I've googled "transgender opera singer" which got me a few interesting articles about the culture, but not too much about the voice in particular, and the few that did mention the voice talked about how the voice can change with hormones. And most of the info that's easy to find is about MtF (FtMs are not popular in the news, for some strange reason).