To get a feel for that look at this picture of my son. He's 6 and not tall for his age. Works great for him. (The CCF pad on top is too wide for him, kind of funny looking on the trail).
Hope your son dosen't get hung up on narrow trails. <img src="/forums/images/graemlins/blush.gif" alt="" /> I have the same setup using an REI UL60. I first carried my pad, (a CSF ridgerest), on the outside of my pack but secured it verticaly. I cut about 2" from each end and trimmed the edges a bit. Now I roll it and place in in the pack and open it up, creating a tube,( a la Phat), that you can stuff everything into. It streamlines the setup nicely and eliminated the stuff sack. I don't know the length of your sons pack but just an idea you might consider. I wouldn't want him to get spun around and fall. <img src="/forums/images/graemlins/shocked.gif" alt="" /> But at least with that setup, if he lands on his back, he'll have cushion. <img src="/forums/images/graemlins/grin.gif" alt="" />