Surely there must be a mechanism in place to deal with these types of situations. There is no way your son is the first to encounter these issues.
As for the warnings, believe me, we are heading them all. DS will have had chemistry and college calculus before attending so hopefully it will give him at least a small foundation to work from. He is not at the top of his class, so getting a poor grade will not rock his world like some folks who have never gotten a B, ever. Although he has friends like that. One in particular got her first B+ (ever) as a quarter grade this year. She lost her mind. Completely inconsolable for days. My son's take on the whole thing was "No big deal. Figure out what you did wrong, learn from it, and move on." Somehow that helped her more than her parents trying to convince her that she could still get into Harvard/Yale/Brown or whatever Ivy Legaue school she plans on attending.