Here is a link to US department of labor information on pre-employment, disability-related inquiries.
http://www.dol.gov/odep/archives/ek96/inquiry.htm
Relevant to this thread, “at the pre-offer stage, disability-related questions and medical examinations are prohibited under the ADA.”
Most other potentially discriminatory questions that are not disablity related, though, like the ones Markxxx listed, are not illegal. However, if he were not hired, the fact that the questions were asked creates a legal exposure for the company. They would have to defend why they asked the questions if they didn’t plan to use the information to illegally discriminate. So many companies make it a policy not to ask those questions, to avoid creating that legal exposure or give the impression that they discriminate. Again, the disability related questions ARE prohibited by the ADA (Americans with Disabilities Act).