It is convienent when they are young for a child to be motivated by punishment and reward, but in the long run no child is honestly motivated in this way forever without it winding up hurtful in the end. Even though you feel especially challenged by this now, be grateful as your child is motivated by something inside of himself instead. Christine Gordon's answer does a good job of outlining the pitfalls of punishments and rewards, so I won't bore you by repeating her clear and detailed answer here. Instead, I will offer up one hypothesis about something that MIGHT be going on here.
You mention that your son gives up if something doesn't come easily to him right away. This MAY be indicitive of a perfectionistic personality. Something that is very common amongst gifted and advanced children but exists throughout the population as well. Because of all the work I have done with twice-exceptional kids (kids with both a gifted area and a learning, emotional, or behavioral disability, I have had a very large number of encounters with perfectionistic tendencies and it is a tough challenge.
Does your son have a specific talent in which things have always just come easily? Is he really hard on himself when he gets something wrong? Is he particularly sensitive and becomes emotional easily? Did he have an easy time in school up to a certain age and then the avoicance of doing his work began? If you answered yes to a number of these questions you may havea perfectionist on your hands. The challenge for these kids is that they are so afraid of messing up or doing something wrong or just not being immediately good at something that they'd rather fail by not trying than try really hard and still not find success.
If this describes your son, you very likely have a tough road ahead, but the objective becomes about convincing him that his effort is more important than the outcome in many circumstances and that there is value in the learning takes place in the effort. You may find my answer to this question also helpful as it outlines a number of techniques you can use to create an effort focused environment and encourage your son.
Include non-evaluative statements (described by Christine) about his effort. "I noticed you got four questions wrong and instead of being upset you went back and redid them."
"I noticed you went and asked your coach for extra advice about kicking the ball."
"I noticed you spent the first 15 minutes while you worked on your homework taking some notes about what you were reading"
I also suggest trying to give him more options and more say in what he does. Let him choose which order he does his homework in and even where he does it (to some extent). By giving up a little control, you are helping him to be a happier, more well-adjusted, independent young man capaable of learning from his own mistakes. I'd suggest Parenting with love and logic to learn more about how to relinquish control while still setting the parenting boundaries you need to set as his parent in addition to the texts already mentioned by Christine.
Finally, Let him fail a little. For example, if he doesn't get his homework done, it will only hurt him. He is eight so better to fail now than when he is in high school and the grades count. Let his teacher have to discuss with him why he didn't get his work done. Let him get a couple of F's for not having done his work and then have to do the make-up work. While he is behind on his work, he also can't go do anything that costs money or have anything new. Don't warn him about this, just let it happen, again, not because he is being punished, but because as a child his job right now is to do his schoolwork and a couple of simple chores. People that don't get their work done, don't get to keep their jobs and when you don't have a job you don't have money to go see a movie, go out to dinner with friends, or by gifts for others and go to their parties. When he gets caught-up he can start doing these fun things again. (That way hopefully, he doesn't fail his grade level entirely).
If he can't keep his room clean - friends can't come over (no one wants to spend time in some one else's mess). You get the idea.