A couple of thoughts:
Just because an assignment is "easy" doesn't mean that a kid will fly through it. In fact, for DS8 it is often the easy stuff that he finds the hardest to get done. Recent example: he balked at doing math problems like (1/4) / (1/3), but was excited about doing (1/4!) / (1/3!).
Is it possible your son is a perfectionist? I sometimes procrastinate because of this. I worry that the work I will do won't be up to my high standards, and so I avoid doing it at all until the last minute. It also takes me a long time to make decisions, and so "easy" projects can take an unreasonable length of time to complete. (For example, we haven't had our house painted in years: what color? what kind of paint? whom should we hire to do the work? what time of year is best? etc. I strongly suspect an NT household would have just done it. Me, I worry I'll regret one or more of my decisions, so I put off making any decisions. Maybe I should be researching house paint right now, but instead I am responding to forum posts!)
None of this is advice, but just know that if there are other underlying issues, like perfectionism, offering rewards and punishments probably won't help much.
