hey, i was in the sasme soert of position as you guys about 2 years ago, only i didn't get money to get a nice bike, i saved 500$ by myself hahaha.
ok, so in MY opinion, that would be the perfect choce for your son. Guessing on he's 10 years old, i'd probably go for the small wheel model, of course, if he's a giant, take the big, but you probably need to go to the shops and have him sit on a bike. His toes should be touching the grond, and have at least one foot down. if he doesn't fit either by just a little bit, don't fret, he''l grow, and by the time he has some bike boots on he will be a few inches taller
As for power on this bike ABSOLUTELY PERFECT! but he will grow into the bike, see, 4 strokes are soooo much easier the 2's. DON"T GET THE KX85 stragtht away. let him learn on this bike, also just because it doesnt have as harsh power, he can still have fun, like wheelies, jumps, stoppies and wheight control and balance. all these things will sort of become second nature while riding these bikes. so in my opinion, this bike will be perfect, but i haven't seen him and stuff.
also, being a decent 140 it should be a great stepping stone to an 85 2 stroke.i had a 125 4 stroke then went to a RM80 and found it not real hard to change over. It will take time to get everything right, try some jumps and stuff, get your tchnique right then move to the 2 stroke, otherwise you will go to a mx track and fall off hard. (i did that, like 2 months after i got my rm i fell off on a jump and broke my collarbone), haha.Maybe before you get a 2stroke, maybe get some coaching, it will definately help.
as i said this is just my opinion, and if you don't like what i said just disregard it
hoped ive helped a lil bit