Existing Eruv & Need of Mezuzah

My son is moving to the 2nd floor of a 2 family house with an existing Eruv. What must he do to be a part of the Eruv? (a Shomer Shabbat lives on 1st floor) Also, what is the size of a room to require a Mezuzah? ( a room large enough for stackable washer/dryer or a person to enter with no machines)

He probably needs to do nothing at all to be part of the Eruv, because when the Eruv is made what is said generally includes in the Eruv anyone who subsequently moves in (Shulchan Aruch Orach Chayyim 366, 9; Shmirat Shabbat K’Hilchata 17, 13). He should verify that the customary Nusach was said when the Eruv was made. When a Mezuzah is affixed to a room that is smaller than 2 meters x 2 meters (= 4 Amot x 4 Amot), no Bracha is made. If the room has an area of 4 sq. meters or more but one of the sides is less than 2 meters a Mezuzah should be affixed without a Bracha. Similarly, a room that serves a purpose (that is not just a passageway) needs a Mezuzah even if it is smaller than 4 sq. meters but no Bracha is made (Hovat Hadar 4,7).