im still really new to knitting, so i cant go on experience to answer, but since her model has a 23" head and the pattern measures 21", wouldn't you actually want a 17" hat for the 19" head?
also, are you measuring not around the forehead directly to the back of the head, but "measured where the base of the hat sits" so if you look at the picture you would measure from the forehead all the way down to the nape of the neck and around that way.
you could swatch up some fingering weight yarn adn see if that will work. if you like, ill help you sort the maths. i would love it if you joined me in this endeavour!
also, if your sons head is 19" the way its measured, and you do need an approx 17" hat, , that means, you need to cut out about 4". paring downt he pattern repeat to 3 rather than 4 means youre losing over 5." bumping up needle size might work, but will it make the hat less warm? thats where we need experts in here.
ingrid! hilde! we need you!