I second the mountain bike-with-road tires idea, for several reasons. He may burn out on cycling in a few months, and he'll certainly burn out on being seen with Dad in the next year or so (don't worry--it's temporary). If neither of those things happens, you and he will have a better idea what he needs in six months or a year than you probably do now, and there's no downside to waiting unless a new (to him) bike is necessary to sustain his interest (that smacks faintly of blackmail, but you do what you can to keep a 14-year-old in the family).
FWIW, I got back into riding at age 44 after a long post-college layoff. My first adult bike was an MB, and when I decided I was really more of a roadie, I didn't have the money for a new one (two kids in college by then). I put 1.4 slicks on my mountain bike and did many 50-60 mile rides over about a year before I finally found a used roadie that fit me.