ps - how old is your ds, and what grade is he in?
One thing I thought of is that you might ask him if he ever has trouble knowing what to say or what words to use when he's talking. You might also give him a "writing assignment" that is similar to something he's struggled with at school and ask him to reply verbally rather than writing it down. If your ds has a challenge with written expression that's related to expressive language, he might also have a similar challenge when speaking. My ds has both, but when he was in early elementary the verbal challenge wasn't obvious at all - he seemed very advanced in his verbal output. When he was around 4th grade though, he started telling us that he didn't know what to say and it became more obvious that whatever was challenging for him with writing assignments was also impacting his verbal expression too. That won't always be the case, but it's something to watch for.
I was also curious re how old he is because... 2 hours seems like a long time to sit and write no matter how easy it is for a child!
Best wishes,
polarbear
