Zoom Zoom, I've never bought skis for a kid/teen, so I don't know how to address your question about size, etc. But, yes, you are on the right track with what type of ski is good. I have Fischer Spyders, which are metal edged classic skis. Personally, I don't like breaking trails, but they are advertised to be able to do this, on a moderate level. Most true back country skis are wider. I once went on a group ski and one of the women had back country skis. She couldn't ski in the tracks, and was frustrated. So I wouldn't buy true back country skis if you might be at a groomed touring center at times. To me, the most important thing is the edges. They help so much climbing and descending.
When I was in the Methow Valley last winter, the leader of our trip set me up with much skinnier, true classic skis, for advanced skiers. Although the conditions were terrible, I definitely felt faster. But, those skis would not work in New England.
I would go to shop at a bigger center to ask your questions about weight, etc. But, it seems like it wouldn't be good to get him skis for a 150 lb person when he weighs 115 now.