So I'm guessing Romulans are forbidden to enlist in Starfleet?
He's a Lt.Cmdr. For him to have that rank, he would have to be in Starfleet for a while already, probably finishing the Acadamy right before TNG started. Having a Romulan in Starfleet at that moment would have been headline news all the time, since the Romulans only came out of self-imposed exile at the end of season 1 of TNG (late 2364).
So it doesn't really make any sense to have a Romulan in Starfleet with that rank. I mean, look at Star Trek Titan, which begins in 2379, just after Star Trek Nemesis. There's a Cardassian serving on Titan, who is a cadet when the series starts, and becomes an ensign during the series. He started on the Academy right after the war I believe. Now, that makes sense, since Cardassia now has treaties with the Federation. The Romulans don't.
Perhaps you don't have to make him an Starfleet officer at all. Perhaps he's something like an exchange officer, part of a program where the Romulan Empire and the Federation are trying to understand each other better, with a few officers serving in Starfleet, and some Starfleet officers in the Romulan Empire.
Thanks everyone for the feedback. It helped a lot especially considering I don't know a whole lot of canon from star trek. I'll post my first chapter soon.