My kitten is suddenly acting sleepy and sad. What’s going on?

Dear Most Esteemed and Knowledgeable Kitties:
Hi I have four 6 mo. old kittens I adopted. Their mom was a feral cat who adopted my son. One of the kittens seems to be sad, sleeping all the time (as of today, and not cat napping). He is not himself: He’s not playing or eating (but he’s [...]