Quoting ★Bαtgirl:" He shouldn't be fraudulently signing a legal document certifying himself as the biological parent of ... [snip!] ... that's just what I was told when SIL had her fiance sign the certificate for his child, and they thought she wasn't his kid..."
Omg so I'm not the only one whose brain went to birth control? Lol.
As for the question, I say leave it blank. If you and your SO ever split up, then he's stuck paying child support for a kid that isn't his. My ex is doing this for his "son" because they signed a paternity affidavit.