It would be a fairly natural reaction to get a little quiet after a traumatic experience. Helping him talk it out and go over things (sometimes you need to do this quite a few times) is a good way to go.

If, after a few weeks he still doesn't feel right by all means go to a doctor. Give him a chance to work through this naturally - drugs would possibly interfere with his brain/body's natural reactions. It's one of the reasons a good dr won't always prescribe Valium for a grieving mother (for eg) because you need to naturally process the events.