Well I don't know how long it takes for someone to get promoted. There's no problem lowering his rank. Thanks for the info Mage!
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.