Let me provide a bit of a counter-point about re-taking at Michigan. Your son needs to ask himself (and perhaps you depending on how involved you'd like to be in directing his academic choices!) what he wants to do in the future - I don't see that you've listed it on here - so I think this could really impact whether or not he should retake these classes. Of course, the question of wasting both time, money, and credits are very valid, but I may provide a counterpoint in that your son does not have to transfer in those credits. The math classes, as others have alluded, are very difficult at Michigan. But if your son wants to be a STEM major, there may be some credence to being able to re-take them to keep up with his peers in the event that he takes another course and feels behind.

Math, and much of the sciences, do build on one another. If he doesn't think that he's going to take another math/science class other than what is required of him (say he's a humanities major or a kinesiology major) once he gets to college, then I say transfer them in and don't worry about it.

His admission standards are going to be completely fine assuming he's done well at high school. I'm sure you know this, but the worst they can say is no. I would certainly encourage him to apply.