Sorry, I need to ask....have you ever had him tested for ADD? (Not ADHD) Do you notice that he will start a project and not finish? Does he follow directions well - can you give him 2 or 3 directions at a time and have him complete them? It's good that you are NOT buying "it's a boy thing!" because, if he's getting tutored and you are not getting results, it's a red flag. ADD can look like laziness. Grounding or punishing him isn't the answer until you know if he is in control of this behavior.
Have you talked to his teachers and gotten their feed-back? Trust me, I hate to "label" a child, but, in my experience, I would rather see a child with ADD get the help that they need, rather than believe that it's laziness. If it truly is laziness....you already know that the sports or extracurricular activities should be curtailed. And, I agree with you about incentives; we don't do an incentive program in my home either - we've tried to instill in our kids that everyone of us is a Representative of our family....everything each person does, reflects back to the family - every morning when they walk out that door - good or bad!
I hope you get an attitude adjustment with your child, soon. Best of luck.