sorry havnt read the big posts above this properly. just a skim, dont have huge amounts of time., but id steer away from the 125 2t as a trail bike, ive been there and done that, and they are fine if and only if you are a confident rider on one , and if and only if nobody is going slow in front of you.
if you get to a tough bit on a 125 and you dont have room to be 'fucken dealing to it', your screwed. as much as it pains me to say this, a good beginners bike for a lighter rider like your boy is something along the lines of a 250 4t. i recently got my mrs a CRF250x (the electric leg model) dropped it down a little and shes getting the hang of it sweet as. her bigest drawback is her lack of height. but at 55kg theres plenty of power there without being ridiculous about it. (personally i find it gutless

)
as for yourself though i will repeat that the cr250 is a serious piece of machinery and something along the likes of ktm's exc's could be the go, i dont know much about them - do they make a 250 or 300 ktm lads? perhaps that could be the go.
i definately agree though with the 2t maintenance. easy as. anyone with some half decent basic hand tools can fix pretty much anytihng on a 2 stroke
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