my 9 month old kitty is acting lethargic kind of??! :(?

 
My 9 month old kitty is acting lethargic kind of??! :(?
So background history...i got my 2 kitties at different times one when he was 6 weeks old and one when he was 4 weeks old. I had my 4 week old kitty until he was 6 months and then we had to put him to sleep because he had FIP and you cant cure that =( that was on christmas day. now my 9 month old kitty is acting like the other one used to act although this cat still eats but im afraid that hes going to stop and he has FIP too. He could be acting like this because we are babysitting 2 other kittens that are 6 months old and hes been running and playing all day and night so im not sure if thats why or not. or if hes just sick?? what do you think? I think im taking him to the vet if he doesnt perk up by thursday but I just wanted some imput to see if anyone has had the same thing?? Hes a healthy chubby kitty so theres no big signs of FIP but im still scared. so imput needed! thanks!
Lets hope you cat can fight off FIP. Keep cool.
OMG!! this is serious u need to take it to the vet right away and try everything there is to figure out what it is!! I <3 animals cats mainly.I had a kitten that was just like yours and was put down!! GO TO THE VET!! ASAP