It's easy to do.  Open a custodial account.  If he has a job (paper route, dog walking, mowing, snow removal, etc.), now is a great time to get him to save 10% for charity, 20% for himself (Plowed into investments--no matter what) and let the rest of the world squabble over the remaining 70%.

My son has had an account since he was 12, checked the market every day when he bought his first allotment of the Schwab S&P 500, SWPPX.  Now it's every few weeks that he checks his positions.

No commissions at Schwab for Schwab Index Funds.  Stock trades are $4.95.  The Schwab app is easy enough for a kid to understand.  

My son has learned more about the emotions of investing with real money on the line than I could ever teach him.  More kids should be in the market.  It's a huge eye opener when suddenly I hear "hey dad, I just got paid a dividend...reinvested in the S&P!".

Slowly he's warming up to the idea of dollar cost averaging, forcing him to buy more when the market is down, less when it's climbing, but buying more all the time nonetheless.  Time in the market trumps "Timing the market".