The employment of the son is none of the employee's (father) business. Sure, we all would like to help our kids when & where we can, but once he makes the introduction between the kid and company, that's as much as he should be involved. Releasing any info to the father could be a breach of confidentiality...potentially giving the kid justification to sue the company...and make more than he would had he been hired.
OP, if you don't want to hire the employee's son...don't. IF the guy asks you why...either tell him, or don't. If he doesn't like your answer, tell him that HR issues are confidential, and that if he doesn't like it, he's free to join his son sitting on the couch while he finds another job.