Involved in the dimethyl sulfide degradation pathway. Catalyzes the oxidation of dimethylsulfone (DMSO2) to yield methanesulfinate, which is oxidized spontaneously to methanesulfonate in the presence of dioxygen and FMNH(2). dimethyl sulfone + FMNH2 + O2 = FMN + formaldehyde + 2 H(+) + H2O + methanesulfinate Belongs to the SsuD family.