Maybe he needs more practice time in a non competition setting. Just because he wants to race, does not mean he should be racing. It sounds like he needs to spend time practicing proper riding techniques and developing bike handling skills before bad habits become ingrained or he gets hurt. Everyone has their opinions on bikes, but none of them mean anything unless the bike you pick is correct for your sons riding style and physical attributes. There are a lot of riders who's style is better suited to a mild powered four stroke. A lot of kids and even adults for that matter tend to be loop out Larry's on 2 strokes because they have poor throttle and clutch modulation and get all whiskey throttled in rough sections and jumps. In the beginner/novice classes the bike choice you make is really irrelivent as not one kid in that class can ride any bike to its full potential. So let your kid ride what suits his style, that he is most comfortable on and dont worry about peoples opinions as he is the only one riding it and who needs to be happy with his riding development. Send him to a MX school for a day or get him some riding technique videos such as Jerry McGraths "Techniques of a Champion" watch them, practice, practice and practice some more until proper riding technique becomes automatic response. Start out slow and smooth with proper techique then slowly work up the speed without loosing technique using a daily training log and a stop watch lap counter to track his progess until it reaches a competitive level, then and only then head back to the track. Just remember most beginning riders want to jump big, but he would be much better off learning good corning skills as there are way more corners on a track then there are doubles and triples. So in closing if your kid is more comfortable on a CRF150 then so be it, you can upgrade to CR85 forks and install a big bore kit as his skill level dictates. Just dont get carried away as I see a lot of people build bikes way beyond there ability and it makes then slower riders overall. Just for a little background I am an intermediate rider my brother is pro and we have been riding/racing motocross and hare scrambles for over 20 years and have personally made all of the mistakes I aforementioned so please do what is best for your sons long term riding development not his short term desires.