I think Coach has a good handle on the situation. So as a high school coach in charge of pitchers. A lefty throwing 78-81 will have success and will get many strikeouts on JV. My goal and approach with him would be to get him in the strike zone more. Because once he gets to Varsity the runners on base will catch up with him, and I imagine he falls behind in the count a lot. Which is going to force him to throw a lot more hittable pitches. I had lefty very similar in every category that you described. he is now a junior. he pitched varsity last year and had little success. He was not use to not being able to blow people away. it was a huge adjustment for him. He is now pitching with a lot more confidence and success. The key for him was playing for a much better travel team and facing Varsity hitters. You will have to increase the level of competition for him. He has to get used to pitching and not just throwing. I have found it is tough to teach kids without failure as to how to pitch.
As a coach for high school, I am looking for kids under 85 to be more contact pitchers and guys over 85 to be more my strikeout guys. This is a very general rule. I have seen plenty of guys be the opposite.
From personal expierence, I was a lefty pitcher in high school. I was 83-85 with a very good breaking ball. I did not want the ball put in play. I walked a lot and struck a lot out. I threw a lot of pitches or did not go a lot of innings. It took me until college before I figured out how to pitch. I got pounded in the fall trying to strike people out. All of sudden I was not good enough to walk 2 and then strike out 3. Now I would walk to and give up a double in the gap or worse a homerun. That failure was the best thing for me.