These are also really good sites and click on the pink tag, orphans, I’ve added in the top right hand corner for loads of previous and good advice on raising kittens. Have you got the right stuff to feed him with? Kitten replacement milk or canned goats milk are best. You can also use kitten glop (home-made formula). I’ll give you the links:
Kitten sites:
http://www.messybeast.com/handrear.htm
http://www.kittenrescue.org/pages.php?pageid=15
http://www.fanciers.com/cat-faqs/orphans.shtml
Kitten glop:
http://www.hdw-inc.com/glop.htm
He probably won’t poop for a bit, with kittens it seems to be every couple of days and he may not have had much in his tiny tummy when you got him. (Also, as the blog explains, you may have to stimulate him although at three weeks he may be able to handle a litter box.) Ask all the questions you want, we are not vets but we will always try to help and we have several kitty foster mums with loads of experience. Welcome to TDK and thank you for rescuing this baby, you are a hero!