Since your OC is a writer, does he have any preferences for where he writes? I think it might be helpful for figuring out a personality. For instance, he could prefer somewhere quiet like a library, suggesting he isn't very social or values focus. If he were to prefer somewhere where he could hear other ponies around him, it could suggest that he has the potential of being a social pony, or doesn't like being alone.
Also, I like your drawing. It's way better than my first! I ended up just using the pony creator game on deviantart.