How can I get my 8-year-old nephew to flush after pooping?

When Im about to enter the toilet there are poop inside after he poops he doesn't flush it. he's 8 years old and I told him so many times to flush its getting annoying now. Its also not nice for someone to flush someone elses poop. No matter what I say he doesn't flush toilet after using.
How can I get my 8-year-old nephew to flush after pooping?
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