Reborn,
Sometimes the $50 is not worth what you have to go through to get it. Sometimes the principle is worth it. You will have to decide that.
Since the parents do not owe you the money, you could get in legal trouble for stalking or harassment. It would probably cost you more than $50 if you are charged.
A polite phone call once a day might do it. After several weeks, they may tire of hearing your voice and tell their son to pay up. Or they might have you charged.
Or call the son's employer. A man owed my ex $150 and when we went to collect, his answer was "Sue me." Wrong answer. I merely showed up at his work daily, inquired about him and was very polite. I also called daily, asking for him. When his boss told me that I was not welcome there, I told him that when I was paid, I would stop calling or visiting, except for my normal business there (I was also a client.). I was always very polite and cooperative. Finally, his boss sat him down and told him that his personal life was affecting his work life and that he had better separate the two damned quick. He paid up.
You have no legal grounds to go after the parents. They have no obligation to talk to you or their son about this.
It might be wise to eat the cost and chalk it up to experience.
Richard