He earned some of this money and some from his bar mitzvah
If he earned it, why not let him decide? Let him read about compound interest, investing basics, and offer to discuss the options with him. Have him calculate what he'd gain at different interest rates.
https://www.investor.gov/financial-tools-calculators/calculators/compound-interest-calculator Let him think about whether he wants to risk losing any of the money he worked for by investing in the stock market. When will he want to use it? How much of it will he need at that time?
That said, I agree with dm123. ETFs are good for long-term investing of money beyond what one really needs, but it sounds like this is his entire savings, so CDs make more sense. Capital One has no-risk CDs with no minimum required, 4.30% for 3 years, a little less for 4 or 5 years. If he doesn't expect to need it all at once, he might put half in 3 yr, half in 5 yr.
https://www.capitalone.com/bank/cds/online-cds/In 3 years, $1200 will become $1342 in a savings account at 3.75% interest, and $1394 in a CD at 4.30%. Some people wouldn't bother transferring to a CD for that $52, but for a 14 year old it can be a valuable introduction to financial responsibility. And maybe even fun.