I would have him cast it 2-handed for this year until he starts his growth spurt and gets some more wrist strength. I taught my kids (2 boys and a girl) to fly cast when they were 5 years old by having them use both hands on the rod and spey casting with an 8 ft 5 wt. When they were 9, I got them casting a 9 ft. 7 weight by spey casting with both hands on the rod because this prevented wrist fatique.
My now 11 year old (his birthday is in August) stated to cast his big brother's (age 16) St. Croix 13 ft 7/8 weight for short periods of time last summer and fall. I am going to get one these for him late next month; however, he has started a pre-pubescent growth spurt and has much more strength than a year ago.
Wrist strength is the biggest consideration for 11 year old and younger kids. That is why I taught my kids to use 2-hands and how to spey cast the single hander with both hands on the rod. They all had no trouble quickly learning how to overhand cast when the were age 8 with the 8 ft 5 weight.