My son and many of his kindergarten classmates also lack motivation to do worksheets. At this point his teacher is handling it. Basically he doesn't get free time until his work is done. Which sounds a lot like life.
I am by no means an expert and my kids are not little saints (and perhaps this is why) but I would try rewarding (AKA bribing) him for timely completion. I'd talk about it the night before and really talk it up. What ever his currency is, use it. It doesn't have to be a toy or screen time. My son is motivated by silly things like getting to dress the dog in human clothes before his walk.
Bear is smart and he's probably figured out that in this situation getting a time out is a win. He wants to sit and not work, if he doesn't sit and work, his punishment is a time out where he gets to sit and not work.
