What changed from K to this year? Maybe the culmination of the work is too much of a jump, even if he is capable of doing individual pieces. Maybe the last two years seemed to be mostly play-oriented, and now things suddenly have a serious tone. Maybe K took 30 minutes, and 1st grade takes 2 hours. Maybe you spent more time working with him in K, but now don’t. Maybe he got to jump on the trampoline more last year.
I think you need to decide the root cause before you’ll know how to handle things.
Also…I noticed that all of your motivational tools mentioned were framed in negative terms (consequences, things to take away etc…) Instead, try to find positive motivators. For example, one I used to use:
“Let’s see who can get done first: you with your math, or me with folding this basket of laundry. Ready…go!”
I was in the room, I could interact, they could see me folding and the concrete model of me getting work done, and I made it fun to race me–we all got our work done. If they finished first, sometimes they’d help me put some things away. And I’d be mock-upset, “What, you beat me?!! How did this happen?” and they would giggle. I made it so they could win if they tried.
If I got done first, I’d say, “Let’s finish your math together.” It was MUCH faster and more efficient for us to take 15 minutes to do math together than for me use up my time and energy trying to get a child of this age to do math on his or her own. We’d snuggle on the couch together and take turns writing, or if I could see they were tired, I’d scribe and let them tell me what to write. A sticker on the page when it was completed (and maybe one to wear on their shirt) was often motivating to my kids. They liked to pick out stickers and I always had a stash.
6 is young, and just because a child is intelligent, doesn’t mean he or she has the mental perseverance and physical stamina to work independently. A lot of 6 year-olds would be doing K this year. Hang in there! Every child is different. Pray about it and see what God shows you about how this child needs to learn and what will help this child to grow. View this challenge as, “How can we learn to walk this journey together?” versus something adversarial.