Maybe he is going through a growth spurt. I have an 11 week old kitten and a 15 week old kitten and they take their moods. Juno likes to play a little and sleep a lot. Percy likes to sleep a little and play a lot. Give Coal a day or two. As long as he is still eating, drinking, and using the box, I would wait and see.
My 14 year old cat recently decided not to eat for a day. I was worried, but I waited till the next day and he started eating normally again. Everyone/thing can have an off day.