Just make sure he has decent standover clearance if you are going with a larger than currently recommended frame. There really isn't a good way to avoid having to replace bikes as a young person grows. Each frame size only works well through a certain height range. You can make some adjustments with stems, setback seatposts, etc. but probably the best route is to keep trading in used bikes as he grows. My daughter is now a young adult, but she went through four different size bikes between the ages of 10 and 18.