A friend is helping with my startup. What's the legal arrangement we need?

My friend is helping me develop my startup. He's doing it on the side and will not be an employee.

What do I need to do now, from the legal side, to protect the company (and avoid red flags to investors)?

He's doing it all for free, and is happy to as he's using new technologies he wants to learn. The classic worst-case scenario is your friend suing you down the line when you become successful.