FWIW, I bought my daughter the smallest Trek Skye; since it's actually a woman's MTB, it's a 26" wheel, but for her, is kinda like a 29'er. She was eight when I got it for her, not particularly tall for her height. Thing that's nice, for her, having a dropped top-tube, the standover wasn't an issue; but this is a bike that will serve her into/through high school; I can raise the seat on it and ride it myself (and I'm 6').
Of course, your son wouldn't want to be caught on a Skye, but the smallest of the lowest-end Treks might work, maybe like an 820:
http://www.trekbikes.com/us/en/bikes/mountain/sport/820/820/#/us/en/model/fit_sizing?url=us/en/bikes/mountain/sport/820/820 Slam the seatpost, and see how that does for him...
On Saturday, September 8, 2012 2:47:47 PM UTC-4, Daniel McClain wrote:
Right, right. Sorry. He's almost six, and exactly 4 ft. tall. Those look nice, but I'm pretty sure he's still in the 20" wheel size.