Among the Enemy by Margaret Peterson Haddix

Book six carries on the story of the shadow children through Matthias, another third child on the run. He is on the run with two other shadow children, who become sick. He leaves them briefly to find help, but ends up saving the life of a Population Police member and becomes the favorite of a high ranking official. He is rescued by Nina and Mike with the help of the perennial character, Luke/Lee. The two children he travels with are rescued early on by Mrs. Talbott.