Under Section 10 of the present rules of the Labor Code,
"SECTION 10. Certification of Employment. - A dismissed worker shall be entitled to receive on request, certificate from the employer specifying the dates of his engagement and termination of his employment and the type or types of work on which he is employed." (Section 10, Rule XIV, Book V, Rules to Implement the Labor Code)."
Write a letter to your previous employer and quote this provision and hope that your company will heed your demand.
Although the Labor Code provides for this said law, employers often, if not always, dismissed such request because the law does not provide for any penal sanction for non-compliance.
There is however Senate Bill 1005 seeking to amend such provision by providing penal sanction (fine not exceeding P100,000.00 and/or imprisonment of not less than 6mos to 1yr). Until the proposed bill is approved and becomes a law, I'm afraid your previous employer will just continue to ignore your request.
But there are other ways to prove your employment. You can demand a certification for SSS payments from your employer. Such certification will include statement you were its employee from the period covered by the SSS payments made. This certification might suffice.
If this doesn't work still, have an SSS online account and you can print out information there about your previous employer.
And your last resort is to file a complaint before the DOLE for your employer's refusal to issue a Certificate of Employment, the agency can better help you with that.