This story reminds me of a young man who came to work at my company. After several months of employment his mother appeared ranting & raving about him not being paid. This wasn't the case, he had been paid but just didn't tell his mother as he spent every penny he earned and more & was seriously in debt with credit cards. This may not be the case with your son, but this does happen.
Another possibility is that his manager doesn't have a clue. Experience of this too I know managers can get it very wrong, so maybe he has been given the wrong information by a clueless manager.
They have to pay him, it's the law.