If you're going to pay the guy full compensation you probably don't need an employment contract. Just pay him for the work he does and stop paying him when he leaves.
If you're not going to pay him, you should cut him into the equity plan
. The Slicing Pie model is the equity plan you and every other bootstrapped startup on the planet should use. It will not only tell you how much equity to give him (and you), but also it will help you determine the right buyout price if he leaves.
Slicing Pie will easily accommodate this: "The expectation is that the work will start part-time, to allow freedom for freelance and project gigs to pay the bills, and as the need and funding grows, the hourly commitment will grow."
In the early days, with so much uncertainty, you need to implement a dynamic equity program that will self-adjust as things change. Slicing Pie is the answer. Send me an email through SlicingPie.com
and I'll send you more details on how it works.