"she told the registrar she had no partner, didn't know the fathers name, and that as far as I can tell, he's telling me that she somehow got his name on after (penned it herself)...I don't know but I'm glad I asked him."
It doesn't work like that, hattieboo.
When you go to register your child, they will write down all the information on a template. They will then show you the information, and ask you to confirm it is correct.
Then, they will enter all the details on their computer and print a draft birth certificate for you to check.
Only once you have confirmed that this is correct, will they get their actual birth certificate out.
The birth certificates are serial numbered, and the Registrar has to document the serial number, the name of the child, mother, etc. The registrar also has to do it for each copy she produces.
Finally, an entry is made in the Register of Births.
If your DP's Ex had 'penned him in' then she would only have been able to do it on the copies of the birth certificates held by her. The actual Register of Births is not in the public domain, so she couldn't doctor that.
If your DP truly thinks that his name was falsely put on a birth certificate, then he can ask to check the original document. It will only have his name on it if he accompanied the Ex and agreed to it.
There is something very, very wrong with this description of events 