"...and cancels everytime I ask him to come in. We only work on Saturday/weeknights"

He has a baby, family, and a house. You refer to him as a "kinda cofounder" paying him well below market rate and have provided him no equity after he's put in 100+ hours into the project.

With all of that in mind, you're still complaining that he does not come into the office on weeknights and weekends?

You need to get real with yourself, seriously.