You can tell him Euphoric's signature:::
WAR ON DRUGS : $500 billion
Citizens arrested: Millions
$ that could be saved in a year if we legalize drugs: $37 Billion
Getting high every day despite it all: Priceless

Hahaha!

But, if it where me in that situation?

DUDE! He's trying to convince YOU... why should you lay everything out for him???

Tell your uncle to go to his computer during the week, do some research... read a lot of stuff, and come back to you the next weekend with the information... that way, you can both discuss what he has found.
Why the fuck should YOU make all the effort? If you're just bound to get shot down...