There are a few skeptoid episodes about it. Maybe point out that once you get past 14 levels of five that's every man, woman, and child on the planet? Try and make him understand that he is the customer, not the people he's trying to sell to? Point out that you have better odds of winning a game of roulette than making money off an MLM?
Edit:
https://skeptoid.com/episodes/4176
Some fun facts:
-The nonprofit Consumer Awareness Institute analyzed available data published by the MLM companies themselves. Of the companies surveyed, they reported the least successful was Amway/Quixtar where 99.99% of distributors lose money, and the most successful was Herbalife, where 99.42% of distributors lose money.
-They also surveyed 200 tax preparers in three counties in Idaho and Utah, where 6% of residents are active network marketing participants. From over 300,000 tax returns, not a single one reported significant profits from network marketing activities.
-In a Wisconsin lawsuit, the tax returns of the top 200 of 20,000 network marketing participants were examined by the Attorney General. The average income of this top 1% was -$900.
-Newsweek found that fewer than 1% of MonaVie distributors ever qualified for any commission at all, and less than 1 in 1,000 recovered the cost of their required monthly purchases.