You should send a polite reminder email or message to your CEO, reiterating the importance of the experience and relieving letters for your future opportunities.
Set a Deadline
In your follow-up, you might want to set a reasonable deadline for when you need the signed documents. This can help create a sense of urgency.
Offer Help
If the CEO is unsure about the format, offer to provide a template or examples of experience and relieving letters from other companies. This could make it easier for him to sign.
Document Everything
Keep a record of all communications regarding this matter (emails, messages). This documentation might be useful if you need to escalate the issue.
Talk to Colleagues
If you have a good relationship with any colleagues or other founders in the startup, consider discussing the situation with them. They might be able to intervene or provide support.
Consider Escalation
If you still don't receive a response, you might consider reaching out to any board members or higher-ups in the organization, if applicable.
Be Prepared for Alternatives
If the situation doesn't resolve, think about how you can explain your work experience in future job applications without the formal letter. Focus on skills and achievements from your time at the startup.
Legal Advice
If you feel it's necessary, you may want to consult with a legal professional regarding your rights and the potential implications of not receiving your letters.