Section 65K. Any employer to whom a policy is issued pursuant to section sixty-five A may appeal, within sixty days after the effective date of such policy, to the commissioner of insurance on the ground that the premium charged upon such policy is not reasonable or is unfairly discriminatory, and said commissioner may, in his discretion, after a hearing of which all interested parties shall have reasonable notice, approve or disapprove the premium charged. In the event the premium charged is disapproved by said commissioner, he shall direct the insurer to which the employer was assigned to issue a policy or to adjust the premium thereof at a rate or rates found by the commissioner of insurance to be adequate, reasonable and not unfairly discriminatory, and the rate or rates so determined shall be effective as of the date of the policy, and be binding upon both the insurer and the employer.