Hello! As long as the canon portrayal is sufficient to depict his personality at that point, it's a valid canon point. In other words, a very brief flashback - a few pages or a few minutes of screentime - would be problematic; the longer and more detailed the flashback is, the more acceptable it would be.

(If you think the flashback you have in mind is borderline, toss us a link, and we'll be happy to give you a more specific answer!)