I decided because many other people put advice on what to do, I'll provide some insight as to why your son may have been chosen.
LHPs are rare as your coach said. I've read many articles about LHPs playing in the MLB, and being equally as good as RHPs who couldn't get past AA ball. This is simply because lefties are rarer, so they have to be less selective. A LHP is very different than a RHP, in the sense of ball movement. They also have an advantage over Left handed batters. The coach may remember a good time with all left handed batters.
Besides this, your son may have superior movement. Some people in the MLB (Chris Young for example) don't throw hard at all, but get loads of guys out (or Greg Maddox!) He [Young] throws high 80s . This is simply because his fastball has excellent movement and hitters have a hard time with it.