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.