What makes you think your son is draftable? Just being a lefty, coachable, smart, and/or a blank slate doesn't. Having a potential MLB tool does.
What have the area scouts during home visits say? Has he been given lots of questionnaires? Have cross-checkers or assistant GMs contacted him? If so, what did they say?
Topping 92 is nice; but what does he sit? Does he miss bats? How many ks/9? How many bb/9? Does he have a devastating second pitch (absolutely needed for a closer) or a third or fourth pitch (needed for a starter)?
Do not for a minute think/believe pro pitching coaches are the true north; some clubs have coaches who can develop, while many others don't. With 60ish new players (half of them pitchers) being brought in each year, there isn't any desire for a 92 mph lhp project. Now, I have seen 100 mph players drafted and kept for years as projects - but not 92 mph pitchers.
He may get a pro shot, but you haven't provided compelling data which indicates (1) this is the year and/or (2) he is slot potential.
Moreover, he has no control over the process EXCEPT for pitching lights out, with repeatable mechanics, showing MLB potential velo and control. When he does that, the next step is have a bonus amount which doesn't self-eliminate.
Do some research on old threads - MILB is a form of hell which should only be chosen with open eyes.