More encouragement -- try for one day, one week, to just praise him and bite your words when you want to criticize him. In the long run, try to convene a negative message in a positive way. For example, instead of saying: you did this (wrong) again, say "I'm sure you can do it that (the right) way."
Next, make his schedule include regular free play time. So that it won't be 反正没有完的. You can start with a few very simple tasks on a weekend, so that he can finish for sure within his limits. Then he can free play. Later, sit down with him and make his task list during a school day -- make sure the planned work time is less than half of total available time. If there's not enough time, you may want to cut down to just school work.
Sometime, less pressure and more encouragement/trust from parents may just do the trick. Good luck!