Will wants to be just like Kalena. He wants to go where she goes, and do what she does. And that includes playing dress up. We are woefully short on dress up clothes geared toward boys (although Will does regularly wear his dragon costume) so mostly he dresses in the same kind of things Kalena does.
So if Kalena is wearing a headband? Will wants to wear a headband. (And a crown. Around his waist.)