I think his decision on whether or not to take calculus should depend on how certain he is about which schools he wants to apply to. There are many schools out there that don't require calculus. For example, none of the Texas schools (with the exception of the University fo Houston) require it, although most of them do want you to take a semester of statistics in its place. And there are many other schools in other states that allow you to do something similar. If your brother knows where he wants to apply, he should contact those schools' admissions offices directly to find out exactly what they require (school Web sites and med school admissions books provide info, too, but I've learned from experience that they're not always totally accurate). Will he be limiting himself? Somewhat. There are schools that won't consider him without it. Someone already mentioned Harvard, and I believe Dartmouth and Duke are two others that also require calculus. There are definitely more. However as I said, there are many schools that don't require it.
Also, keep in mind that while some med schools may not care whether or not he takes calculus, his undergraduate college may. For example, my school's physics sequence for premeds is calculus-based, so I don't really have a choice. He might want to talk to his undergraduate advisor before deciding one way or the other.
