An interesting question. At first I didn't see the distinction you were making, but now that I do, in a society in which literacy is probably not as common, it's a fair question.
What do you think, given what you know about Eothain?
At 14 Intelligence he's well above average, so there's that to consider certainly.
There's also proficiencies that I think could/should factor into it (i.e. basically any "knowledge" proficiency would arguably indicate the ability to read).
Ultimately, I'm not too interested in depriving you guys, as players, of information that adds to the narrative and setting. So I'm pretty willing to say, "Yes, you can read (and write) anything that you can speak."
But if you guys are interested in something more nuanced and possibly more realistic, I'm fine with that too.