Assuming he didn't live in the district on election day, he is not eligible to serve. The Board of Elections cannot certifiy his election and the City Clerk cannot swear him in. Moving into the district now does not rectify the problem because the legal eligibility requirements were not met.
So what to do? There is yet another vacancy. The Mayor should call for another special election, Mr. Euguene should make sure that he lives in the district on the new election day.
This is such a shame because his victory is of such significance. But if he won this time, he will certainly win again.