Strictly speaking, once you've tied the older game to your steam account you can't really give it away through Steam (as I am sure you know).
While I have no knowledge of the extent of Steam integration a retail version of New Vegas possesses, the following article claims that:
Steamworks is implemented in "as light and unobtrusive away possible,"
and it will be mandatory to play the upcoming Fallout. That said, you
can install the game on as many systems as you want, and besides the
initial activation, an Internet connection isn't required.
Assuming the above is true, you could install the game on your son's machine, activate it one time using your account and then he should be able to play without needing to connect to steam. Mind you, that just because further authentication isn't required, it doesn't necessarily mean that the game won't try to perform it anyway.
This is all assuming your son has his own machine, of course.
Also, as Juan Manuel pointed out in the comment to your question, sharing your steam games is prohibited as per their terms of service. Seeing how New Vegas simply relies on Steam for DRM and you are talking about retail copies this may not be the same case, but it's something else to consider.