In the last week, I've seen two MomoKai stories that used "taller boy" and "shorter boy" to differentiate between the characters. Only they didn't agree about which was which.
This sort of thing might work for Hagrid/Dobby fic, but Momo and Kaidoh are too close in height. (I think Kaidoh is the technically taller, but Momo's hair sticks up pretty high.)
Here are few epithets you can use instead. It's clip-n-save!
|purple-eyed boy||dark-eyed boy|
|spikey-haired boy||bandana boy|
|Dunk Smash Specialist||Boomerang Snake Expert|
|easy-going boy||uptight boy|
|hamburger addict||training freak|
|boy with the annoying voice||boy with the sexy voice|
Or you could just use their names.