The hard part here is convincing the teenager in a way that makes it seem like his idea/decision. Remind him that HS ball is about winning for your school. It's not travel or showcase baseball, it's team baseball, school and community pride. That a good left hand bat is a weapon for the coach and team. Why deny them of his talent to help them win the division, state's, etc. Plus, ask him if he really wants to sit waiting for his turn in the rotation or stint out of the pen when he could be contributing to winning games. HS Coaches will often ride their seniors or #1 or #2 arm depending on how the schedule shapes up or the weather impacts it, if he's not that 1 or 2 as an underclassman, he could be lucky to see double digit innings during the season. Take the field, the HS career is short.
IF that doesn't work, wait till his HS girlfriend says she wants to go to his games. He'll find a way to play the field.