anniversary/restaurant week/need restaurant
I have been to Fleur de Sel during RW, they served their braised veal breast (their signature dish) and skate as entrees. It was a pretty good experience. Other restaurants that are romantic or has really great food that is participating in RW (but most likely have no more available seats) are Asiate, Aureole, and Chanterelle.
Since most restaurants limit their menu selections during restaurant week, no matter where you decide to go, it would be a good idea to call and ask exactly what the choices will be so as to be sure there are non-fish options. As for the allergy to nuts, of course, you will need to discuss that with your server to be sure none are present in any dish your bf orders. All this said...
Fleur de Sel, one of my favorites, would be the perfect place for a celebratory lunch. Although I've never done RW there, I have had many of their year round $25 3-course lunches, all of which have been delicious. The small space has attractive decor and pleasing ambiance.