He was wearing someone elses face. Like he did to Arnold.

What if Arnold's head went to a place where other kid's heads and body parts are; if you look at some of the other monsters: one has a porcelain doll head, some have stuffed animal heads, Arnold got a pumpkin, some have arms/bodies that are stuffed animal parts. As Boy wears the skin out/gets damaged, he replaces them with the parts Snik collects to maintain his childlike appearance as he lures other kids into the world.

Since Arnold still looked like a little boy, a some point, he could use Arnold's face. Boy with blonde hair?