Unless there is a lot more to this, he should be refused a seat.
I am not sure what the mechanism for replacing a an election Victor during the congressional interregnum, but if there is no federal law, state law, or election law, the Republican leadership should tell him to get the hell out of town.
https://www.foxnews.com/politics/new-york-rep-elect-george-santos-confesses-lies-about-his-resume