I am pretty sure he already knows that. During the ghost arc with him and the other teachers, all of them were talking and realized that they have met before. Ebisu has already outed Sano as his brother in the train arc, so I am sure Haru has already figured out that was Sano.
this is It Haru will Know that Sano is the Mii-kun He meet when he got Lost and gave him the Charm omg. but we will wait for a Month though.
I am pretty sure he already knows that. During the ghost arc with him and the other teachers, all of them were talking and realized that they have met before. Ebisu has already outed Sano as his brother in the train arc, so I am sure Haru has already figured out that was Sano.