Sorry to hear of his problem, however this can be a common problem with thesse hand reared ones.
Are you still having to stimulate him to go to the bathroom by wiping his back end with a damp cloth? If you haven't tried this then it might be worth doing. Sometimes using a rectal thermometer as if you were checking his temp can also stimulate them to go ( don't insert it far, just the very tip). One of my receptionists rears a lot of our kitten that get dumped with us and she will often add extra water to the formula to try and get the stool a little softer ( don't add too much or he won't be getting enough nutrition). Have you started to offer real kitten food yet, Sometimes a little canned kitten food can also help things along.
If he really isn't pooping anything then he may need to be checked over by your vet who may even need to give him an enema to get things moving properly.
good luck
Receive up to 75% off pet meds at entirelypets.com