Good Morning and welcome to Just answer. I am Dr. Loretta, a licensed veterinarian and I am happy to answer your question.
From what you have described, it appears that Benny has injured himself at some point in time while running anf playing. This is not unusual for young kittens as they race around. Since Benny has trouble with his rear end, this could be a fractured pelvis and this is treated with 1-2 weeks of cage rest and, since he is so young, they will heal on their own. It is best to have your vet xray Benny to be sure this is an injury that does not require surgery. They will prescribe an anti inflammatory medication ot help Benny through this.
In the meantime, make sure that he is eating and drinking...if he is not , give him a dose of Nutrical every 4-6 hrs. This is a high calorie supplement available at the pet store and this will give Benny the nutrients he needs to get past this.
Does that make sense to you?
Please let me know if you have ANY other questions. My goal is to give you 100% satisfaction and if you are not yet satisfied, please Reply so I can clarify for you
Sincerely, XXXXX XXXXX