It is definitely possible to close that gap from 7.5 to 7.0 (or even lower) in a 3-year period.
Most 14 year olds are not actually in that great of physical condition. They don't make a habit of working out yet. So just trimming down and getting stronger helps.
Getting sound instruction from a running technique instructor is much more helpful than many realize, provided you're dealing with someone whose record indicates they actually know what they're talking about.
But between 14 and 17, you'll find out whether this is something your son cares about enough to do the work. Most don't and therefore don't see the improvement. The good news is, that tends to clear the path for those who do.