This is incorrect... many states do have laws making it illegal to sell or furnish eCigs to anyone under the age of 18. (The OP's home state of California being one of them)
HOWEVER.... the 0 nic issue, is an interesting one. I do know many of us planned to use California's legal definition of eCigarette as a loophole to SB648, if it passed, since it seemed to require the presence of nicotine.
Personally I'd still recommend against it... Sure Vaping may be less dangerous than smoking... but that's a pretty low bar to clear, so to speak. Why risk it?
Now, if he's going to do it anyway... Maybe you could at least point him in the right direction; quality products, american made liquid... or better yet, since it's 0nic... might as well make his own... it'd be SUPER cheap and easy.