HELP! im raising an orphaned baby kitten and it wont poop!?

hi, a firend of mine's cat had a litter of kittens and rejected all of them, some died but this one lived and ive been bottle feeding it and trying to stimulate his rectum so he will poop, and he does a little bit but it seems to be hard and its very little at a time. also, when i feed him he seems like he is starved to death yet he eats healthy about ever 3 hours which is what my vet told me to do was feed him every 2-3 hours. ive tried wiping his with baby wipes, warm clothes, everything. does anyone have any suggestions? please any ideas are helpful.