Fit should be the decision maker! Make sure to try on as many as you can... ideally under load.
Ion'e Echo seat harness in size XS fits a 25-29" waist and SM fit a 28-31".
Dakine's SM seat harnesses fit 28-30". Something like their XT model would be worth a look. If your son is skinny and/or on the shorter side, Dakine's Vega harness may be a better fit.
Prolimit's XS seat's fit 27-30". They also make a harness that's a hybrid waist/seat that may be something to consider. Mystic also makes a hybrid harness that may work (Star Hybrid).
Mystic's XS seat's fit 25-30" & SM fits 29-33". Check out their Aviator model.
Some women's models may also offer a good/better fit, so don't be afraid to give that a try. The only issue may be the colors and graphics.