I can use that income to contribute to the Solo 401(k), and use that money, along with whatever money I decide to roll over from my existing Traditional IRA into the Solo 401(k). 

The online version makes it absolutely frictionless to work together on your bookkeeping with someone wherever they are.

Could he rollover his IRA to a self directed IRA?  

@Toni Bandyyou need a self directed IRA or self directed Solo 401k if you want to purchase RE.You will pay no taxes if you do a direct rollover. 

When you quit your job - you will be able to rollover your 401k into self-directed IRA and invest in rental properties or other alternative investments. 

On top of that they recently completed a major AirBnB update that allows the dynamic pricing to roll over to all their partners instantly (as before there was a lag and lots of issues syncing up).By no means and I trying to talk you into Evolve as I'm guessing it comes down to the 10% (actually it's 12.5% when you include payment processing), but I will say it's worth evaluating what your time is worth to see if offloading those highly time consuming and convoluted tasks might be worth it, and you can spend more time expanding your portfolio.To compliment my Evolve services, I use digital door locks and some other tips and tricks you can read here.

Wouldn't it be better to have this note with a 1-year balloon with an option to roll over the note for another year?

You are right, once they are in an IRA you can do it, but I think the discussion point was the difficulty in moving the 401k money into a Rollover IRA so that it can be placed into a self-directed IRA.The reason employers do not support individuals rolling their money into IRA's is because the employer benefits from from keeping your contributed funds to your 401k under its umbrella.

Digital contract signing.You want to create a frictionless selling system and then build out frictionless onboarding and fulfillment systems.

Based on the information provided, it seems like your lease will go will roll over to a month to month unless you have notified your tenants correctly that they lease is ending.