I know he is only 8 but I would begin to teach him about having faith in himself and his decisions and taking pride in himself.
I admit, we lay it on a little thick to our kids, but we have told them...
"You are only as good as your word. Your actions and your words are the only thing you have to offer, they represent you, all other things can gotten elsewhere. If we can't believe your words, we can't believe much else about you, either."
Most children lie to get out of trouble. We let the kids know, everyone makes mistakes, it's normal, and an acceptable part of learning and growing. If they make a decision that turns out to be the wrong decision, they must "own up" to their mistakes, and set about correcting them. If they lie about their choices, they will be in twice as much trouble.
Each time he lies, I would express my disappointment in him, and he would be disciplined, but I would keep it simple and not over react. To carry on about it too much would be giving him too much power. Also, make things make sense... if he lies about a chore or making a mess, make him complete the first task, (the one he lied about) then add another, similar chore as a punishment. (A child that throws food in the cafeteria should not be sitting in a classroom for detention, he should be cleaning the cafeteria.)
In most situations, people either choose to act or re-act. Try acting instead of re-acting.
Example of re-acting - Finding out he did not complete his homework after he told you (lied) and said he did. Now, of course, this needs to involve punishment, and yet, the homework still isn't done.
Example of acting - Sitting down with him, going through his backpack and finding out what he has for homework, (or checking school website or making arrangements with teacher to have assignments emailed, etc.) Send the message that school is important, and homework comes first. He will not be allowed to play video games, watch TV, whatever, until his homework is finished. The homework will get done with no chance to lie about it.
(For homework - ) Play around with the schedule and see what's the best fit for him/your family. Around here, if they have a game/practice, they must do homework as soon as they get off the bus. On free nights, we give them a choice of doing it as soon as they get home, or right after dinner. Actually, while I'm making dinner works out the best, as I'm am right there to assist and stay "on top of them," if need be.