All romans are called "Firstname Romanvs". If that fails "FirstnameA Romanvs" all the way through to "FirstnameZ Romanvs".

So he'd be "Servivs Romanvs" and the command:
give_trait "Servivs Romanvs" [whatever the trait is]

Course if he's not the only one, chances are you simply can't give him the trait because they may all have the same internal name.