As an employee, your boss cannot do this.
First thing to do is consult a lawyer for advice under Slovak law. I live in the UK, but my sister-in-law is a lawyer in Slovakia and I know that the laws can be different there. Depending on the wording of your contract, your boss might not be able to use your code, because you will own the copyright unless he follows the terms of your employment contract.
Secondly, look for a new job. You do not want to work for a boss like this.
