Catherine, I am curious if your son has synesthesia. If so, he'd have specific colors and possibly textures assigned to each number. For example, the number two would be blue. Even if it's black text, it'd be blue for him. A lot of 'numbers' people have this, and it gives them great joy to examine numbers because of this.

Echoing others, 25 % sounds fine to me. If I were you I'd pursue his passions right alongside him, as you are, then he is certainly being social smile