"When we started I didn't know his abilities." - why not? This makes me question you, not him. Were you a poor interviewer? Were you inexperienced? Who signs on

"When we started I didn't know his abilities." - why not? This makes me question you, not him. Were you a poor interviewer? Were you inexperienced? Who signs on with a co-founder without knowing what the other person brings to the table? Did you not look into his prior work, interview employers, etc? You also don't give me enough background about you/him/situation to give me any insight that could be truly helpful. Are you a great programmer thus able to tell that his output/work is not up to snuff? Or are you the business guy and just unhappy that he's not giving you what you want as fast as you want? Hell, for all we know he's kicking ass working through some insanely hard problems that no one else could have done in half the time.