If he has good command he’s a potential D3 pitcher. At upper 70’s the best hitters (D1 players and prospects) would stand in the front of the box and dare him to try to throw a fastball by them while looking for the off speed stuff.
If he’s getting hitters out with upper 70’s stuff in high school I have to question the quality of the competition. My son played in a 6A program. He considered 80 slow pitching. He rarely saw it. It was mostly 85 to 90+.
Lefties usually get a little benefit of the doubt on lower velocity. But I’m not sure about down in the 70’s.