Dear Aziz,
Like in aesthetics, where beauty is in the eye of the beholder, in employment, ethics is in the attitude of the contracting party.
When a prospective employer requires you to disclose details of your previous employment, it is your duty to provide them accurately, without discretion for any reason.
Similarly, when the particulars of previous employment provided by a new employee are not fabricated but merely incomplete, the employer may view it as an omission rather than a lack of ethics warranting termination with stigma. The employer's refusal to provide an experience certificate adds insult to injury. However, one must remember that in an employment contract, the employer is typically the superior party, and employees should be aware of this in their pre-employment and post-employment transactions.
Therefore, in my considered opinion, you have two options: One is to seek an audience with the CEO, request leniency as the omission was unintentional, and either continue your services or resign gracefully with a service certificate. The other option, if the first proves futile, is to wait for termination and pursue legal action under the ID Act, 1947, if you are a workman, or otherwise, file a civil suit to declare the termination orders null and void. This second option may be a long legal battle, provided you have the time and resources.
My suggestion, therefore, would be to resolve the issue amicably to maintain your employability.