My 42" ds is just getting too big for the 12". I'm planning on getting a 16" for his 5th birthday next month, but that size is still a bit big. 14" would probably be perfect for my ds this summer, but too small next summer. There also isn't much selection in the 14" size. Evidently, it is a much more common size in other countries (Canada and Great Britain) judging from my web searching.
If your ds is average height like mine, I would try to get a 14". the only reason that I am planning on getting the 16" is that he is a year older than your ds, comfortable on a bike with training wheels, experienced in braking, good at hopping off if the bike tips, and relatively cautious. Just make sure at least the balls of your ds' feet can touch while seated, but flat footed is best for a new rider. A lot of the sizing info will say that the child should be able to straddle the bar with flat feet and an inch of clearance. But young riders don't have the presence of mind to hop off the seat to get their feet on the ground so I wouldn't go by that.
Mom to unschooling 4everboy since 8/01