Portraying aphantasia on the page

I'm moving on to the last of my 4 POVs now, and I'm sort of stumped as to how to convey to the audience that this character has aphantasia, without just outright diagnosing him with it. Particularly as this is a pseudo-medieval setting. It is sort of an important plot point, and I want it to be clear in the readers heads.

Now, visualization is an important part of my magic system, so naturally this means that he would be unable to access that magic system. I have tried already to have scenes where he attempts and fails spells, but do I need anything more than that? Are there everyday things we all do that would be impacted by this condition? Things to keep in mind when portraying his thoughts and views of the world?

Thank you in advance for any suggestions!