Also, because he didn’t have sex doesn’t mean that he didn’t contract it orally. MG is often asymptomatic in the throat. You need to take an extended course of azithromycin, if that fails, take a long 10-14 day course of Moxifloxacin.
MG tests are incredibly unreliable. It’s odd that symptoms would not show for that long but 6-18 months before symptoms is common. Either he had an asymptomatic case for 3-4 years which is unlikely. Or he is not being truthful and had an infidelity after your November MG test. What type of specimen did you send in to get tested?