Just do it subtly over time, start asking him thought provoking questions first about things he brings up or things that are current, and when he's comfortable questioning authority then you can start pushing other things. A really good, subtle YouTube channel and video series I think you could also recommend is "bigger than you know" by the Company Man. It's basically some 30 year old amateur who makes these 15 minute videos about business and companies' histories and rises and falls and such. The Bigger Than you Know series really shows how much companies have grown through acquisition over the last 25 years, though, and demonstrates how much of our consumer economy is owned by the same folks. It does all this from a totally neutral, even normie perspective, but the implications are clear if you watch, and it's probably the exact right kind of thing for a 15 year old to take to.