You know my opinion b/c we've discussed this off-line
. I think it makes sense to develop a change up. 1) it makes the fastball better (effectively faster), _if_ the arm slot and arm speed are the same as the FB. 2) it can also have a lot of movement (often 2-seam tail action) with work and refinement. 3) at some point, especially if your son doesn't end up throwing 95+, your son will be thankful he worked on it.
A power pitcher as a Freshman...he may end up gaining another 10mph on his FB, or he may not. No one knows for sure. But I wouldn't classify a HS pitcher as a true "power pitcher" unless he's hitting 90+. Of course, a lot of that depends on the competition he faces (regardless of school classification... even 5A can vary a lot). 83-84 may be blowing it by one team he faces, and a BP fastball to another team.
My opinion is to work on command within the strike zone (and just outside of it) with the FB, choose the best breaking pitch that he throws to focus on, and then learn command of an effective change-up.
The pros who do not have change-ups have either incredibly explosive and moving fastballs, and/or they are just 1 inning pitchers. Pitchers who face an order 2-3+ times through in a game, need a change up, IMHO. Some days, if hitters cannot catch up to the FB... may not throw it much, and certainly not in the strike zone. But against the teams who are loaded with high-end talent and can time a good fastball, it will be a more effective pitch than having 2-3 different kinds of breaking balls b/c a good change up has good movement as well as a change in speed.
_All this being said_... there are plenty of very good, college scholarship-pending, HS pitchers throwing just 2 pitches. But that doesn't mean they aren't working on a change-up. We just may not see them throwing it much.