I used to walk a mile and a half to the park, do pull ups on the monkey bars, and walk back. It wasn't really part of my workout regimen, it was just my idea of fun at the time.
Get in the gym. No more theory. Just do it.
He needs to do some resistance training, he doesn't necessarily have to do it in the gym. If he's a private person, and considering he's on the small side, I can understand why he might feel self conscious training in public, especially if it's alongside a bunch of ripped dudes. That may not represent the healthiest form of thinking, but I can certainly understand it.
At his size, he doesn't need to be squatting 300 lbs. right away to see some gains. If he wants to do it, he'll do it. I'm not convinced he wants to do it, but if he wants to, he will. Half the battle is getting started. Once you pull that trigger, lifting can be quite addictive.