I would talk to the teacher and see how he is doing during the day. It 'may' just be transition from home to school that is stressful. (I have a sensory DD that does not do well on any transition...but is fine once the transition phase is over). We role play a lot and I have picture lists of what we do in the morning (get dressed, eat, etc) and that helps her a lot.
Mostly, she does not want to stop playing in the am and go---but once we are there she has a lot of fun and enjoys it.
Since it is 1/2 day- I would try to stick with it a bit longer and see if it improves. 1/2 day K is a nice transition into all day 1st grade. Often kids in K go through a 'No school' phase after a week or two in. The 'newness' has worn off and the routine may be tiring- they are still figuring out friends and rules, etc....it is stressful, but temporary.
Or maybe afternoon K would work better if he is not a morning person??
Also try more sleep--again, my sensory DD does not do well w/o sleep. She needs 11 hours solid (and she is 5 in a few weeks)- her twin needs 10 hours. Both are grumpy monsters w/o sleep and the first few weeks of school we had an early early bedtime to make up for it.
If you check with the teacher and he is struggling or he still does not want to go in a few weeks- you could consider homeschooling. BUT unless you plan on homeschooling for K-5 or longer, you *may* have the same issue next year (if you do K or try to do all day 1st grade).
: I hope it gets better soon!