"Entering" the draft

Do players need to do something specific to be draft eligible or are they are eligible based on age? There was a boy last year who signed w/ a major D1 and wasn't drafted, someone mentioned it was possibley because he didn't "enter" the draft.
2 Scouts have called here (not visiting, just looking for some additional info) and my son recently got a medical history forum in the mail from another. We really don't think he will be drafted, but is there something that needs to be filled out or anything "just in case he has a hot Senior Year and hits 100MPH?"

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"People ask me what I do in winter when there's no baseball. I'll tell you what I do. I stare out the window and wait for spring." Rogers Hornsby, Hall of Famer