You describe yourself as the "product owner". And the dev has no equity. Maybe he feels he's doing all the work and you gain all the benefits. You should offer him some equity, ownership is a great motivator.
I tried offering equity and a slightly lower salary (20%-range lower), and then an option for no-equity but higher salary. He took the higher salary option, though I was hoping he would take the equity option.
I could revisit this with him...but I have a feeling it's not super important to him, money doesn't seem to be a huge motivator.
Exactly. Every startup I've worked for the owners have this weird attitude that their equity is worth something. It's not. It's worth jack shit until the company is worth something. It doesn't pay bills. It doesn't let me buy the new shoes I want. It's essentially an investment. Usually if someone is in the position to invest, they don't need to work a day-job to cover bills.
Otherwise he's sending you a very public signal that he's checking out. Or he's bored, and bored people quit:
http://www.randsinrepose.com/archives/2011/07/12/bored_peopl...