Hi Chris, I'm new to BP and wanted to chime in if you don't mind.
I think that encouraging your nephew to contribute to his 401k as much as possible will be a great piece of advice. Because he's so young, a few thousand put in there over the next few years and left alone for the next 45 years will leave him with a nice nest egg, no matter what happens as life provides its lessons and hard knocks. Key point: you'll want to have him think of it as money he can't touch not as a loan source.
I agree with Bill that his savings rate should be crazy high right now with you guys subsidizing his life so generously. So he can put money in his 401k AND save for his first property. I'm saving up for my first ever property, a duplex or other small multi, and I'm putting my money into the Life Strategy Income fund from Vanguard:
https://personal.vanguard.com/us/funds/snapshot?FundId=0723&FundIntExt=INT
Minimum is $3K so he may have to save a little first, but so far it's treated me well. It's mostly bonds with about 20% stocks to give it a little boost.