BK35- you've gotten great advice here that should be helpful. I want to add 2 points:
Once students get to college and take some basic classes and pre-recs they often change their majors. So like they say here on the hsbbweb don't pick a college strictly for premed (or baseball).
I have worked many years with medical school students. They all had outstanding grades in college and great MCAT scores. Medical school is very selective and in my neck of the woods only the top students get into good med schools. One woman majored in a more difficult major than biology @ UC
Irvine. She had 2 B's mixed in with all her A's so not a 4.0 college GPA. She was not admitted to the UCI med school but strangely enough was accepted by Stanford and is doing great. If you don't go to a well respected med school it can be tough to get a good residency program.
But if your son is really motivated, gets to college and does do the premed route you may find his college grades may be better than high school due to maturity and focus. Good luck.