How old is your son? How aggressive is he?
When my son was 9 I was showing him things, waiting for him, coaching him. By the time he was 13 we had similar talents in many things. By the time he was 16 he was coaching me, waiting for me. Now that he is 20 I keep reminding him that I have NOT made my will out yet so he best not kill me yet!
If I am correct, your son's bike is a 4 stroke so I would recommend that you go four stroke as well. If nothing else it means only having one gas can instead of two.
The next question is if you want a "MX" bike or an "off road" bike. The MX bikes are built for racing; they have better suspension and higher performance engines. Downside is that they cost more and the engines can be a bit more finicky.
The "off road" bikes are usually a bit heavier, engines detuned (which means they last longer), some include headlights, kickstands and other luxury items. There are some "off road" bikes that are pretty close to the "MX" ones, however, which I think might be a good bike for you.
I would recommend something like the Yamaha WR250F. If you buy it used, perhaps only one or two years old you will be under your $6k figure. It will have enough power to keep you ahead of your son's 125 and at least up with his next bike ( I assume that he is still growing and will be into a larger bike in a couple of years).
Rod