check the sizing chart:
Just to verify. When you mention the 16" size (S), that equates to 41cm. So 41cm seat tube length (dimension A) would be the S size with 27.5" wheels. But.... What is the inseam of your son's pants. You want your son's stand-over (flat footed measurement from the ground to the crotch) to be around 2 or 3" more than the bikes stand-over dimension (dimension L). Then with your son's stand-over requirement, you may have better luck searching for the bike. You may find that if your son is tall, a medium size bike would fit better. You really don't want a really long seat tube sticking out of the bike frame. And maybe the medium size would work better has your son is still growing.