The only feasible way to do this is for you to run a "synthetic exchange." A few hundred dollars at any brokerage will lead to HIGH monthly fee's and trading expenses will all but make it an exercise in futility.
I like the investopedia idea. You can track his total return and pay out his % return on his capital. So if he has $500 and can generate an annual return of 10%, you pay him the $50 when he closes the position. Only downside is you'd have to pay him his return. Or if he loses 10% take that out of his principal. Much better learning environment than giving E trade most of his money in fees and expenses.
My son is WAY too young at this point but I'd like to do something like this article illustrates when he is older:
Fee Creep and High Interest Rates at the Bank of Dad - Bloomberg