I think the "standard" advice on this forum for people starting out in investing is to open a brokerage account with Vanguard and buy VTSMX (Vanguard Total Stock Market Index Fund Investor Shares).
If you go with Fidelity, I would personally start out with something on
this webpage.
There are several things to keep in mind. One is the "Expense Ratio". That's basically the cost of holding the account. Lower rates are better.
Another is commisions, otherwise known as "loads". I personally like "no load" ETFs or funds. This is another expense to try to minimize.
Also, it is probably best to avoid buying individual stocks unless your son is willing to put in the time to learn the companies he's looking at buying. It's better (less time an effort) to stick to index funds.
Finally, there is the question of what to do with earnings. For example, what to do with dividends. If this is a long-term investment, I would choose to "re-invest". That means that the earning will automatically go towards buying more of the investment funds.
