call teams and volunteer your time. Start at the bottom, sweep, clean mud off the cars and clean the tires and trailors. Be the guy who comes in early and leaves late.
I did that one season. I was lucky the first team I asked took me on. We worked after hours most nights. I guess it did help that the team owner knew me for many years and knew my karting background and knew I wasn't a flake. But I found out, I didn't like the long hours, travel, etc. and wasn't what I wanted to do in the long run.
Goodluck to your son
Does he want to travel ie WOO ? Talk with crew members and ask what life is like on the road.
It's not an easy job, especially if he wants a career.
BUT DON'T LET ANYONE DISCOURAGE HIM, if he's got the drive, he will have a good chance to make it.