Hello, as a licensed veterinarian I am happy to assist you!
How wonderful that you are taking care of these kittens! I am sorry to hear that one is having some diarrhea.
First let's start with some feeding tips:
I recommend feeding a formula specially made for kittens, and at this age 4-5 times per day if you can manage that. A great formula is called "kitten milk replacer" or "KMR," which can be purchased from any pet food store or even amazon (link below). Make sure to follow the instructions that come with it for feeding (for refrigeration and warming instructions, etc.). Second, at this age these little guys can start transitioning to canned food: buy a food formulated for kittens and mix it with a little water. Then leave it out for them in a shallow plate/dish and let them eat it as they want to. Also always make sure they have a shallow bowl of water available to them at all times.
KMR on amazon
As for the sick boy, this is potentially very serious. There are a multitude of possible underlying causes for his diarrhea, including things like parasites and bacterial overgrowth (common in outdoor/barn cats), which would require treatment by a vet. Additionally, diarrhea in these tiny guys can lead to life-threatening dehydration very quickly. For these reasons I highly recommend he be taken to a vet. If the other kittens have not developed signs yet, it may also be worthwhile separating him from them in case it is something contagious that they have managed to not catch (unlikely at this point to be honest so keep a close eye on all them). Until you can get him to a vet make sure he has water available at all times and continues to take his milk replacer. I do not recommend attempting any medications without a vet seeing him first.
Thank you for your question, I hope you find this information useful. Please do not hesitate to contact me with additional questions or concerns. Once you are satisfied with my answer please remember to rate my answer so I receive credit for my time. Thank you and all the best to you and your new friends! ~Dr. Drea