+1 to Rick's commentary. I've ridden my brother-in-law's GG250 several times and although the power hits harder than it does on my 200, it's very tractable.
I'd say that if your'e willing to invest the time in getting the jetting sorted, as well as the suspension bits, it'd be a great bike. More than likely, your son is going to be amazed at how light she feels in comparison to the CRF150. Not that there's much different in actual weight (no clue - maybe there is...lol), but where she carries the weight makes it awesome on the trails.
And the front end is planted; biggest thing that sold me on my 200 over the KTM 200.
I'm 5'10" and going on 44 - cockpit of my 200 is perfect for how I ride (trail hack). I bet your boy likes the bike once he adjusts to the power delivery difference from the 4T
