Homeschool and Family Economy

How do you motivate kids to do their school? I implemented time blocks into everyone’s school day. More structure for those who struggle getting it done and less structure for those who do well budgeting their time. However, my 8 year old son will give me 2 days and then days 3-4-5 will cease doing any math or language arts. I have already decreased his load and his goal is just finishing 2nd grade, which he started last year, instead of moving onto 3rd. I figured maybe he just needed a less busy year with more time to play, but now all he wants to do is play and now he may not even finish 2nd grade math, of which he was well over 1/2 done when we started the school year! We have implemented into the Family Economy that the youth cannot have any internet time or read “twaddle” until they have put in a good day of school and completed chores, but he doesn’t seem to care. At this rate he will enter high school at 18. My 15 year old son has become behind before, and so has the 8 year old, eventually they catch up, but this time the 8 year old is so far behind I have not even added any new lessons. I make sure a lesson takes under 40 minutes and do not assign every problem. We use Christian Light, if that matters. Anyhow, any tips on using the Family Economy or motivating him? He also does not get to test for his next karate belt or go out with friends until he does a good day of school. In the past, I would try to wait him out getting frustrated and using every threatening, manipulative, and bribing tactic I could think of out of desperation. Eventually he would do his school. When he felt like it. At 8 years old I don’t think the talk of “why school matters and we will love you even if you live under a bridge” is appropriate. I learned with my first son that boys can sit for hours at a table and do absolutely zero school work. Their minds take them on incredible journeys just sitting. I could have made this more concise, but typing on a phone lends itself to rambling. Sorry about that. Any advice appreciated. Oh, he does play a lot during his allotted school blocks. Should I force the issue of bringing him back to the table to finish school?