I haven't looked to see what position the son has at his father's business or the nature of the business itself so I can't comment on specifics. I will say, however, that it isn't uncommon for parents to employ their kids under the table for below market wage commensurate with their lack of experience. If a job gets half done for a quarter of the cost (no taxes, benefits, etc) then for many people it's a win, not to mention that the employer/parent has a captive employee and the employee/kid has early job history and walking around money.
With either type of arrangement, I think it does tell you something about the parent-child relationship, which in turn does influence how you should take the parent's testimony.
rather at the expense of the business owners, which may well be the parent themselves? (ok fine the other employees could have equity in the business, but other than that)