Are you open to indexing to start? I'm a proponent of stock picking, but it takes time and immense effort. I think your son would benefit greatly from indexing and working on learning stock picking via paper trading.
If you aren't putting much in and adding in smaller increments, look into Fidelity, specifically FZROX. Fidelity will allow partial shares and there are no fees or minimums for that fund and some others. The downside of Fidelity's FZROX is the lack of history (compared to VTSMX with Vanguard, but it requires a $3,000 minimum). Full disclosure: My children invest in FZROX as part of their allowance/financial "education". The lack of historical data on FZROX is a large downside to me, but I'm going to give it some time to prove itself.