Even if I plan to potentially come on full time and take a significant equity stake, I've learned (the hard) way to always charge for my work. Equity is essentially meaningless before a certain point in the startup's lifecycle. While this potentially allows you to get in early on in the startup, a number like 5% means that the guy doesn't really value the equity all that highly yet. Considering that a lot of founders don't give their first employee 5% (and that vests over 4 years), he either thinks that you are awesome, or doesn't really know what he's doing. I have seen new startups bring an advisor on at 5%, but that implied A LOT of work, vested over several years, and it always seemed like they were being too generous.
If the company is structured as an LLC, it probably isn't going to be a standard startup. If you want real investors, you have to form a C Corp (since there are too many restrictions on transfer of shares in an LLC).
In summary, I would take cash, and setup a contract for 100 hours at most. Feel free to take a significant hit on your normal consulting rate, but charge him something. Also structure the contract so that there are well-defined milestones, and you get compensated as you hit the milestones.