What do you do if you fail to register your child's birth? Especially if he's a few years old already. How can you rectify it?

You can go to this site: https://psa.gov.ph/article/delayed-registration-0 and check out the requirements that are needed to get a certificate of live birth. Alternatively, you can go to any NSO satellite office near your location and they should be able to help you out :)