The 105 would actually land you in the Supermini class for MX, I don't know about harescrambles. An 85 would be a more natural progression. An advantage of the green 85 is that the parts to upgrade to KX100 specs are available (although probably expensive). I do no know the KTM line well enough to comment on its upgradeability. My current intent is to move my boy to the KX85 in 2-3 years, depspite the fact that I think it's an ugly bike.
At his age, what bike does he want? Good for him for shunning the 150, and noticing the weight. If you can get him a little riding time on that 105 before commiting to any choice, you might be surpised by the result. My son thought he wanted a KTM 65 a 1.5 years ago after riding a mod bike in a field. Fortunately for my wallet and gas tank, he was allowed to ride a friends stocker on an MX track and didn't like the hydraulic clutch. He chose to stick with Kawi.