They don’t have to poop every day, Samantha, every other day is fine, especially if he is weak and still on kitten replacement formula, you wouldn’t expect a lot of poop. Click on the pink ‘orphans’ and ‘constipation’ tags for many previous threads on this subject. This set of videos is also great. http://www.youtube.com/watch?v=ctfhMJUdxZA
I am worried about his lack of strength. There is a special supplement called Nutrical for kittens that you can give to sick, malnourished and underweight kittens, you can buy it at a pet store or from a good website like entirelypets.com. I would recommend giving him a little of that, smearing a little on his mouth (no big blobs, nothing he could choke on). I would also make sure you always have honey at hand to smear on his mouth if he seems to crash (go limp and unresponsive), as he might be prone to blood sugar level crashes, and this can be life-threatening. He needs either formula or honey (if he won’t take the formula) or Nutrical if this happens. And if you are really worried, you can try a second vet’s opinion. It might be better to tube-feed this little one until he is a lot stronger. But it sounds like you are doing everything right — best of luck with this tiny one.