Can a highschool senior apply directly into nursing programs at specific colleges? Or must he apply regularly?

Unfortunately, probably not. Most credible nursing programs are very selective in whom they select and typically only admit a limited number of people. Your first step is to, of course, get accepted into the college or university of your choice. The second step is to follow whatever guidelines said college has in regards to its nursing program, which almost always consists of passing a high-level Chemistry and/or Anatomy and Physiology course and having a specific GPA (at my alma mater, the minimum was a 3.5).

Perhaps the admissions requirements aren't so stringent at a hospital-associated nursing program. After all, the main difference is that at a hospital associate program, you will not be spending the bulk of your time in liberal arts courses. You may want to investigate those as an alternative to a four-year college or university.

Applying regularly means repeated applications. Are you planning on not being admitted into any program? Apply directly. Go to a college and talk with a counselor and find out what you'll have to do. Then do that.

Yes you can, but only if you have some sort of work based program at your school. There are schools that offer them. its where you eave school and go to the college where you earn college credits toward a degree and earn high school credits at the same time. I did it. I graduated 2006 with degree in medical assistant and high school diploma.

While you may make some inroads that will ease your acceptance into the nursing program, you will have to gain general admission to the college itself before any program lets you in.