Funny, I came on here thinking about a similar question - my son, 13, has been asking a lot about investments, stocks, etc. (not at all because we talk about them - he seems to have a natural interest in making money!) and I've been thinking of coming up with a way to give him a chance to practice investing. However, he would balk at the idea of investing all of his money! So I'm curious if you've talked with you kid about the potential risk and lack of liquidity. That is, what if he finds something he wants to buy?
I say this because I think it's an interesting challenge as a parent - how to balance teaching savings/accumulation, and just reasonable consumptions/spending. I find that I have to sort of encourage my kids to spend their money because sometimes they talk themselves out of buying something they really want. I want them to have a balanced relationship with money, be willing to make mistakes, over spend on something they don't end up liking and learn a lesson.
I don't mean to hijack the thread, but I think this might all be worth considering before he puts 100% of his net worth in the market. Unless, of course, he has a really big allowance/side gig, and will replenish his liquid cash quickly :)
P.S. the idea I offered my son (after trying to explain index funds and stocks, bonds and money markets, and if he agrees to complete the Khan Academy Finances curriculum) was to let him pick something to invest $100 in. I said I'd match his investment with $100, as long as he leave the money invested for 12 months, we'd see what happened, and it would all be his. He'd have complete control over what to pick - I think he's leaning towards Apple :)