When do you add bok choy when cooking stir fry
Bok choy should be the last ingredient you stir in; it is the ingredient that take the less time to cook.

but, 

I would do it in two steps.

Stir fry the bok choy first, set aside and stir fry the other ingredients and add the bok choy back at the end to bring it back to heat.

The reason I would do it like that, is that bok choy will release lot of humidity when cooking, you do not want to start steaming ingredient.