Yeah, I looked into GD, but their posts require more exposed seatpost than all other brands that I'm aware of. Their 3-inch versions require 6.25 inches of exposed seatpost in the up position. That's a minor detail for adults, but he's about 2 inches short of that at climbing height. His feet wouldn't even reach the pedals with 6.25" of post above the seatpost clamp, ha!
That is close. By the end of the summer that might work.
I'm interested as well ... I have a 27.2 GD knock-off (T-MARS) on my XC bike but it's awkward (takes a lot of effort to release) and too long and he can't use it....he's also tried my RS stealth but he can't depress it with his weight regardless of tube length...
Thompson make a 27.2 but its $$$ ... and I'm not convinced he's heavy enough so didn't explore further.