You are eligible to get relieving letter the notice period salary and all other benefitts which has to be given by your employer.
At first, you can send him a legal notice to grant you the relieving letter. If he does not comply with it, you can proceed either legally or by explaining the issue to your new employer. In case you want to proceed legally, then you can file a complaint against him to either deputy labour commissioner or labour commissioner of your district.
The labour commission will acknowledge your complaint and send a notice to your employer to issue relieving letter. However, if your complaint is not resolved by labour commission then you can file a case in labour court under Industrial Disputes Act, 1947.
The Industrial Employment (Standing Orders) Central Rules,1946 state that Every permanent workman shall be entitled to a service certificate at he time of his dismissal, discharge or retirement from service and the mployer of the establishment shall personally be held responsible for the proper and faithful observance of the standing orders.
They will have to give relieving letter as per the rules. If you do not wish to proceed legally, then you can also approach to your new employer and tell that your previous employer is not granting you the relieving letter. You provide your new employer your resignation letter and an affidavit and undertaking that you have no liability towards your previous employer.
Reference: The Industrial Employment (Standing Orders) Act, 1946