J.C.
I have two suggestions -- either of which MIGHT help - or might not.
Try a ''penalty job''' --- lets' say the 8 year old misbehaves and 'messes up'' an hour time for the whole family. So he then owes the family an hour penalty job. It could be vacuuming, washing counters, scrubbing the floors- picking up pinecones in the yard- raking grass.
Then -- if he spends 30 minutes doing what YOU two parents agree is 10 minutes of work- he still owes the family 50 minutes before he is back in possession of tv time- computer time- etc. --- I also with my middle child- said ''sorry, Daniel-- if the 60 minutes of vacuuming is not done by 5pm tonight- then you will owe us TWO hours- because I don't want you to drag this out. ''
My other invention was a 'penalty letter' == I required the child to write down what happened- what mistake the child made - and what he can do next time to avoid the problem.
Just keep at it-- you are the adults and the source of everything he wants - -hang in there.
Blessings,
J.-- aka- old Mom