I have often made reservations with points for family or close friends. Call the Rewards line, tell them what you want to do, give them the name, and they'll set up the reservation in your son's name. I think there is an age minimum (21?) and he may need to present a credit card for incidentals.
The free night certificates may be different. I've never used one without being present at check in. I'm sure you'll hear from others on this.
It's true, the free nights are different. You must be present at check-in. I've often thought it would be nice to be able to let my kids use these in my absence, but it is not possible. One idea: have your son sign up for Marriott Rewards and then transfer the points to him.