What are the future plans? Trail riding or racing? If yo think you will be going racing then something like a little KTM racing 50 might be the solution. If you plan to just ride for fun then something like a PW or XR might be the solution. Since kids really vary in size I'd take him to sit on a few and see where he falls. My son was always very small when he was younger so we had some challenges to deal with tall seats etc... but a little planning can make those easy to resolve.