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.
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...
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.
If it wasn't your company, would you "spend" $15,000 on the equity you offered?
Would you recommend just granting some equity to try and engender the "ownership" aspect?