When my dad moved to Florida last year, he left his mountain bike behind -- no mountains down there! It's a Giant Rincon with thumb shifters and three chain rings in front, seven in back.
My dad is a retired engineer; he cared for it well. He is also 6-foot-2 ... so this bike is suited to someone of similar height.
The snow is going, spring is on the way -- it's perfect for your lanky teen!