Been there... For my son K was only half day. What I did in this situation was decided not to sweat it since DS was learning other softer skills such as social skills & EF skills. As long as the class is play based and he doesn't seem bored I wouldn't worry too much this year & just after school him more challenging material if you can.

For my son 2nd grade is where the boredom really set in and became a problem.