When should kitten eliminate by himself?

Hello. Last week we took in a small kitten my coworker found in her driveway. Here is a picture:

http://i185.photobucket.com/albums/x.../IMG_1307.jpg?

And another:
http://i185.photobucket.com/albums/x...MG_1264-1.jpg?

We are not sure about the age - his eyes are open, he has cut a few teeth and his ears are starting to stand up. He also is toddling around, but he's no where near old enough to be seperated from mommy under normal circumstances. We know he's had a lot to deal with, because he had two abcesses (one on his neck, one on his shoulder) from where our vet thinks a tom cat tried to kill him to put the mother back in heat - he suspects that's how kitty got seperated from its mom. But the kitten has been thriving - he has gained weight and is always eager to bottle feed. We are feeding KMR. The problem is, he still is not pottying on his own. He has no trouble going when we stimulate him; he goes right away and pee and poo are the right colors. All the articles I've read on development says they potty by three weeks, but I think he's older than that. Our vet says he will eventually grow into it. When should I be concerned? TIA