Hi! I think I can respond to this question as both an OT and parent.
I'm assuming you are concerned regarding the mechanics of his writing skill, not the content. How does he do at the second grade level? Are his skills age appropriate or do they lag behind? I would first address that by doing either formal or informal assessment of fine motor and visual-motor skills. Sometimes adaptations such as pencil grips, keyboarding, dictating or some direct OT can help support the mechanics of writing.
My other thought is why can't just the reading aspect, or those aspects he excels in, be accelerated? I understand that the curriculum addresses the all aspects, but can't parts of it be pulled out? Couldn't he read at his current level and do projects or tests related to that only, if his spelling and writing aren't quite there yet?
It certainly is not unusual to see only one or two aspects of a child's development be so accelerated. I know that others who are active on this board have children whose language skills exceed their fine motor/handwriting skills. I'm sure they will offer suggestions from experience!
