Thanks, everyone, for your comments. The problem with my son is that he has a 24 inch waist. The small men's Dakine is a 28-30 inch waist and the Murray runs big, only comes in medium or large, and costs $184 with bar. There is nothing designed or sized for kids, an admittedly small market segment.

Mike, thank Julie for showing me her Tempest harness. I bought a used Tempest in women's XS that includes the spreader bar and paid only $70. If the hook on the bar does not work on cats I will ask Brian to perform his welding magic to reshape it. Otherwise I will try an old square plate hook.
Cheap extra small used gear