Well, first your son should contribute enough at work to get the company’s matching contribution. That is the most sensible path. His 40l(k) contribution is tax-deductible.
Then if he has “extra” money he can open a Roth IRA at Vanguard or Fidelity, and just make automatic contributions every month. Remember, he doesn’t get a tax deduction for a Roth IRA — and he must have earned income to contribute. I’m sure that at age 24 he is within the Roth income contribution limits.
Congratulations for having raised a “saver”! (Now just make sure he doesn’t marry a “spender”!)