I wish you had asked this question five years ago — because at his age, and depending on his needs/circumstances, I would have suggested that at least half be invested in the stock market for the long run. I would also suggest that some of this money be place in an Individual Retirement Account where it could grow tax-deferred. He can contribute up to $6,000 if he earns at least that much.
BUT, I hesitate to “prescribe” based on this small amount of facts. What your son (and likely you) need is a meeting with a FIDUCIARY (no ripoffs!) fee only financial advisor. You can search for one at
Wealthramp.com — and be matched with a carefully vetted advisor who will ask questions about your entire financial situation and make personalized recommendations, probably using Fidelity or Vanguard mutual funds.