Employer not accepting my resignation

As you are employed in a private company, the RTI law will not apply. No one can compel you to stay against your will in an organization, if you don't want to. If your boss does not want to accept your resignation, you are obliged to ask for a written explanation as to his peculiar conduct.

Send an e-mail to him giving him reasonable notice of two weeks time to find a replacement for you. From your side, the formalities are complete from the time you give him prior notice. If he does not give you the experience letter, that's fine; however he is bound to give you the release letter & your last payslip/paycheck, including all/any outstanding amounts due to you as arrears from the previous months. If that still does not work, send him a legal notice. That is sure to give him jittery feet & make him think twice about restraining you from leaving your job.