My suggestion would be to see if the school is sponsoring any sort of "job fair" or talk to the counselor about possible options.
I would strongly encourage him to begin taking basic classes like English, Algebra, History, Foreign Language, Biology, etc. to get them out of the way, then he can "concentrate" on whatever he decides he wants his major to be.
Auto mechanics can make good money. See if you can take him to an auto-shop and talk to someone who does that for a living. Then find out what about biology interests him and help him check out options in that realm (doctor, chemist, biologist, whatever). That may help him make some decisions. You know something about the medical field and can help answer questions or guide him in that realm too.
I've worked at Caterpillar (Large Engine Center in Lafayette), and they're very skilled workers there who make good money. There's nothing wrong with putting engine's together.
I've heard an expression to "be the neck that turns the head". You don't have to "make the decision for him" or force a decision on him, but you can give him things to think about. Show him what the jobs will be like, and see if that's really what he wants to do.
Most people change their major's many times during their college experience, and he'll likely be one of them. BUT, if he can get some of the basic requirements out of the way while he's considering his options, then the last couple of years he can focus on learning those things he really enjoys whether it's science or auto-related.
Just some thoughts, for what it's worth, and good luck!