Orphaned Kitten not Pooping

I rescued a 2-3 week old kitten from a homeless man on Saturday, June 29th.  The kitten was covered in fleas and I don't know when the homeless man got it or if it had eaten at all with him.  The kitten has taken to the bottle and has a healthy appetite.  I have been wiping his bottom with a moist wash cloth to simulate the momma cat licking and he has been peeing after almost every feeding - and I have found some wet bedding so he is peeing on his own.  However, he has not pooped since I have had him and I don't know when he pooped before I got him.  When should I start to really worry / should I take him to the vet?

When you potty him massage his stomach first. When I have constipated kittens I will sit with them for a half hour and just massage their stomach in a circular motion- he may pass gas and then after massaging try stimulating him. Make sure you are getting his rump area with the damp cloth. Start massaging his stomach for a bit before you potty him every time.
Also you can start adding more water to the milk replacer.
Kittens don't usually poop a lot when they are on the bottle but if his stomach is looking distended or abnormally full there's a good indication that he has to go but is having issues. It's very common for kittens to have constipation or sometimes diarrhea when switching from moms milk to formulated milk since they aren't the same- it's great that he is eating well and hopefully he is just a little constipated from the recent milk switch.

Thank you so much!  I have been massaging his tummy, but not for as long as half an hour.  I will try that, and dilute the formula a bit the next time he eats. I'll let you know what happens!

Ok, if he still has not gone he really  needs to get to a vet so he can get an enema. There are certain things you can add to his bottle but at this point if he hasn't had a bowel movement he needs to have one ASAP.

Let me know how he's doing. 

Some people will say to add ONE DROP of vegetable oil to ONE FULL BOTTLE of formula. Personally I don't really like this and am not sure what others here think of it, but I have used the tiniest pinch of Miralax added to a bottle and that has been much gentler on their little tummies than just plain oil. Still, it's always best to consult with a vet so he can give you the proper doses. 

Just adding more water to the bottle may end up doing the trick, just be sure if he hasn't gone yet he gets to a vet for an enema and then you can ask the vet about the above-mentioned things to prevent this from continuing to happen.

Best of luck with the little guy!

I was worried that she (the vet has now said she thinks he's a she) wasn't pooping so I took her to the vet on Wednesday. She (of course) pooped as soon as we got there.  The vet de-wormed her and otherwise she seems to be healthy and thriving.  She comes to work with me and I feed her throughout the day - all in all, not a bad life for a little kitty!  Thanks for the advice

Haha doesn't that just figure! Glad to hear she dewormed her sometimes that can even cause some constipation ( but many times diarrhea instead). Again, best of luck and keep up the good work!