What is a good way for a beginner to start investing?
Since you are clueless about the subject and only have $100, now is not the time to start. You could purchase books about investing and both of you could study it, but there is nowhere that will open an account that small, especially for a minor.
Educate yourself first. Its not a good idea to just jump into something that is putting your (or Sons) money on the line. Learn some things at investopedia.com and nasdaq.com. As a starter, know that it takes much more than $100 to get into this ball game. Try 1-5k to start. That is as long as there is no large minimum to start using an online trading site. Read and youll learn. You should be happy and proud that he wants to start growing his money at such a young age. Hes made a smart decision there. Good luck to you guys.
The historical compounded average annual return of the U.S. stock market has been about 11%. If a 12 year old invests in an ETF, like VTI or VOO, it should grow to 100*1.11^53= $25,242 by the time he is 65 years old. If he spends the money, it will grow to $0 immediately. https://investor.vanguard.com/etf/
real estate guy
Even if you invest in stock market, the most you would usually earn is around 8%, (or $8). At this age, I would focus on working to earn money vs playing the stock market.
He needs to be taught how to SAVE before how to invest.