I hope this message finds you well, present circumstances excluded. I am a licensed attorney with over a dozen years of employment law experience. It is a pleasure to assist you today.
Unfortunately, unless you have reason to believe that the real reason the guy gave you a bad reference was due to your race, age, gender or disability, there is nothing that can be done here. If you feel as though this is rooted in discrimination based on at least one of the four aforementioned factors, then you can file a claim with the EEOC that could lead to a federal law suit in which you could receive damages.
If there is no discrimination here, as unethical as this conduct is, there is no legal cause of action that is really worth pursuing. There is a cause of action called tortuous interference with business. However, that cause of action and this particular set of facts do not mesh together well enough for me to advise you to pursue it.
I wish I had better news for you. You are obviously a hard working person that just wants to better yourself. It appears that your former employer is unethical, at a minimum. However, unethical alone is not legally actionable in most cases.
Let me know if you have any other questions or comments. Please also rate my answer positively (three or more stars) so I can receive credit for my response.
Best wishes going forward!