If your son has been attending Gatewood then he will be grandfathered at Gatewood and should not have to change schools. Our daughter is in 4th grade also and is being grandfathered at her school. The school district is allowing the children who are currently attending a school to be grandfathered to the schools highest grade, the new plan really effects students and families of kids entering kindergarten who have older siblings at their preferred school, kids entering 6th and 8th grade are also greatly affected by this new plan.
Seattle Public Schools has a link on their web site to lookup your school assignments by your address: http://www.seattleschools.org/area/newassign/address.xml