Sure, he will outgrow it quickly, IF you give him plenty of opportunity to ride it. BUT, he will then be hooked & have a hunger to ride that will be insatiable. As I have posted here before, my just turned 11, 59.75 inch, 80 lb. daughter still prefers to ride our XR 70. She is quite capable of riding the 80, but I see no reason to force her. I figure she has to learn to make her own choices and soon enough will want the extra power and size of the 80. To me, confidence and fun are what riding is all about. I've seen too many new riders get scared and then become reluctant, hesitant, and lacking confidence. That feeling is NOT fun.
Buy the 70; it won't cost you much to trade up to a bigger bike later & you'll know what kind of rider he is and what kind of 2nd bike he needs (playbike 4-stroke or 2-stroke MX.) :ride: :cool: