We did very casual ec starting at 7mos, and dd really picked it up at 13mos and then again about 19mos. Remember, ec isn't all or nothing - it is a gentle, child-centered approach to help kids learn how to meet their elimination needs. There is a yahoo group for late-ec. If you have the potty, and he lets you know, or you notice, just take him to the potty, let him finish, dump and flush (my dd's fave part!). I think every pee in a potty is normal, not a "success" and misses or pees/poos in the diaper are just misses, not "failures". If you have a postive, normal attitude about it, so will he! Good luck!
