Your son sounds like a great kid. Relentless perseverance. I love it!
His play this Fall will speak for itself. Not sure what strategy could be employed. If he's really improved to a decent ballplayer, it will show and the HS coach won't ignore it. All coaches want to win - if someone can help them win, they will be on the team unless they have other issues (grades, behavior)
Most coaches love effort, even if from less talented or experienced players. Most likely your HS coach does too.
Let your son play and it will take care of itself. In the meantime, it sounds like he now enjoys baseball - thats the best thing going here! That will hopefully last for a lifetime. So why not get him on a rec team where he will play and enjoy it for sure!?
Best of luck.