My first thought about your concern about why have doubts surrounding trust and how to work through it was that it was his mom telling you about his cancer and it not coming from him directly. Which I understand and see your point as to why he wouldn't have, it's that in and of itself that can feel like a hurt/rejection of sorts, hence that on the fence sense of insecurity that has come up. Which is rather normal to feel.
Yes, of course that feeling will applify past wounds, but in my initial reaction to reading your post, that's what comes to mind. Seems so minor writing it, yet, even thinking on it It's much more complex than these words that I am writing.

Next step is having a game plan for these months apart so that both of you maintain a level of connectedness. Might be worth talking over with him and finding a compromise. His lack of response isn't helpful.