His chores shouldn't be incentive based. This creates backlash later on when you have them negotiating with you for money or gain instead of Cooperating. Scale it back. Video game / tablet time decreased increased based on performance which is still incentive. Or maybe just explaining that rules are rules and some things just need to be done for life to be balanced. Remember as the parent you're driving the car and aren't paying for them to ride along.
check this out Jules sent it to me and its sort of relevant.
I was trying to comment the other day but my phone lost signal. If I'm not mistaken what you are doing is positive reinforcement "Positive reinforcement refers to the introduction of a desirable or pleasant stimulus after a behavior. The desirable stimulus reinforces the behavior, making it more likely that the behavior will reoccur." He may always expect to be rewarded for chores if you continue this method. Perhaps balance it out with "negative reinforcement" taking away something he likes doing until he does the chore. (Taking versus Giving)