Let me ask you a few questions. Do you have dirt bike experience? Do you know the difference in a 2-stroke and a 4-stroke? If you do not know please educate yourself and TT can do that in an hour. Determine where and what kind of riding he will be doing. This will help you determine the bike- motorcross or trail or combination. Then let me suggest that you go to CraigsList and see what is for sale in your immediate area. This is to match the price you have in mind for your son's bike and what is available in that price range. You then post a wanted note and ask if anyone in your area would let your beginner son take a ride on some of the 4-strokes mentioned on this thread. You see, only by watching your son's reaction to the bike will determine what he is capable of doing. A small 2-stroke (85) has a lot of power and is comparable to the crf150 in speed but is much more powerful. As mentioned before in this thread- fun speed is faster than scared speed. Height is also important! He will want to touch his feet to the ground on both sides of the bike. On the other hand, do not buy a bike that is too (small such as an 4-stroke crf100) He will grow and it may save you from buying another bike in 2 yrs. Let me clarify this point. Skill can make up for a lot of mistakes- too heavy, too tall, too fast, too powerful. A beginner does not need all these adversities. I would not recommend to anybody that their child take on a bike that is too powerful for them to "grow into" but a bike that is too small is not fun for long. Dad, it is up to you to do your homework (you are at the right place for great opinions and suggestions) but they do not take the place of riding the bike. Go to a local Atv park or where dirt bikers ride and ask for a quick lesson or a ride. Learn what these bikes will do! Get your son on several bikes before you buy and you will buy him the right one!