If it's solely based on DNA evidence and you have no paper trail, you can describe the DNA evidence (how closely you match shared relatives, etc.) and any circumstantial evidence that supports the claim (e.g. that he was in the right place at the right time). That's how I handled it for my mother's unknown father, who I discovered through DNA testing. Feel free to look at her profile if it would help. (It's somewhat overcomplicated because I initially figured it out by tying together third and fourth cousins, so there was a lot to explain.) Also check the WikiTree help files- there's a standard format to use in the biography for DNA confirmation here, and if you leave some critical stuff out it'll get flagged.
