my 8 year old son keeps clogging the toilet by whipping himself with excessive toilet paper, what should i do?

my niece used to do that - in the end her mum started making her unclog the toilet herself, after unclogging it twice she never clogged the toilet up again

I know this is now a fortnight old, but nobody seems to've given you a workable answer yet, although the shock of unclogging the toilet can help 8 is a little young for that treatment.  By all means let him watch the unclogging to show cause and effect though.

This is a pretty common phase with boys around this age.  They seem to develop a real horror of not being clean and accidentally touching poop.  Are you mom or dad?  Best would be for dad to have a talk about wiping technique because boys are starting to become hyperaware that it's different for them and they really want to grow up into dad.  If there's just mom available the talk has to be had.  Likewise a refresher talk on proper butt washing at bath time.  If your household doesn't use a squirt gun or bidet having flushable wet wipes available/and or a dedicated, lidded wipe trashcan as applicable, along with a talk about using ONE wet wipe for a final clean up can alleviate anxiety about leaving anything behind. 

Finally, how is your boy's diet?  Boys are more prone to bowel complaints than girls for some reason.  Are his motions comfortable?  Using excessive toilet paper can sometimes be a symptom of trying to alleviate the feeling of "not done yet" caused by constipation.  Less sitting, more fiber, a lot more water, and learning "good elimination posture" (you can google for this, it'll be safe for work) will go a long way to solve this.

Make him UNCLOG the toilet a couple times and you will be amazed at how little TP he uses after that

whipping.........................

buy a bidet like a normal person

make him unclog it, then he'll learn not to do it