Hello, everyone! I have a 6 week old kitten, almost 7 weeks now, that is NOT pooping!

He hasn't gone for almost 2 days now... He eats just fine, and pees normally, eats 4 meals a day...but still NOTHING. I have tried rubbing it's back from head to bottom, and rubbing his tush with a warm damp cloth...still nothing. He also has been sneezing quite a bit, and something that sounds like a slight cough.
If this continues into day 3, I will have to call the vets. Any ideas on how to make him poop and/or clear up his little lungs and nose?
Thanks!