ETF for an 18 year old

Hi all I have a son who wants to do long term investing We are looking into ETFS and I’ve had some good feedback and read through many posts

The main one it seems is “VOO”

I know it’s best to diversify but when I see so and so etf mentioned not all of them are available on the platform I use

Is just having “VOO” too risky?

Outside of that what other stock would compliment that?

Don’t want to double up on another etf that’s simailr

Any feedback welcome