Our son had a particularly cruel prank pulled on him at work. His manager (restaurant) secured a pregnancy test from his sister in law and brought it to the restaurant whereupon he asked/instructed our son's ex-girlfriend to tell him that she was pregnant and then produce the EPT as proof. He was very upset and went into a state of depression even after being told it was an April fool's day joke--some 30 minutes later while he was trying to wait on tables. His mother and I are wanting to know if this would constitute 'harassment in the workplace' or 'sexual harassment'. Thoughts?

No it isn't sexual harassment. It was unprofessional and a bad idea of a joke, but not illegal.