I've got three boys that started flycasting between ages 6 and 8. They each had a tough time casting single-handed with the 9' 5wt. rods but they did great speycasting. No fancy rods - they just grabbed at the top of the grip and the butt and could cast all day.
If you guys do end up shopping for a rod, I recently saw a new one by St. Croix that might be appropriate depending on his age:
*New for 2002 - Model #PF766 features a smaller grip for smaller hands — ideal youth model.
hit the specs link
All the best,