I think it's really about what the bride and groom prefer. In the formal photos, it might show the difference if the FOB is in a tux but FOG is in a suit. Maybe the married couple will pick up the tux rental for FOG. Couldn't hurt to as your son. We dressed both FOB and FOG in tuxes like the groomsmen and groom but it was a last minute decision. Outdoor wedding but reception at a fancy hotel; hence the tuxes.