Either "Gutten har på seg sokkene sine" or "Gutten har på seg sine sokker" would work.
Gutten means "the boy" (
gutter would translate to "boys"). Because the owner of the socks is the third-person subject (i.e., "gutten"), we have to use a
si- possessive. "Socks" is plural, so the correct possessive to use is
sine. We have the option of putting the possessive either in front of the indefinite plural
sokker or after the definite plural