Okay. This definitely helps paint a more clear picture of the situation.

First off, the situation with the interns sounds like you have a few issues. You're trying to turn your developer into a manager, which is fine if he's on board with that, but if you're trying to build an army of interns under his direction, you and he have to come to an understanding that he's going to be acting in both developer and managerial capacities. If he's not on board with that, you're boned. At small scales, the interns, being subordinate, need to conform to the project lead's schedule within reason. Email and IM are wonderful tools and can be used to time-shift conversations, as well.

Second, being underproductive is a very real and very bad problem. There's a glimmer of hope, though. Was he always this bad, or has he deteriorated? If he's deteriorated, it might well be due to a hostile work environment - nothing shuts down introverted creative types like an abundance of criticism and an absence of praise. If he feels that he's the constant butt of complaints without any recognition for progress he's making, he's gonna get very demotivated very quickly. A tech founder in a startup has to love what he does to succeed. If he doesn't, you'd better be paying him damn good money to keep him motivated, or it's a lost cause. There's too much blood, sweat, and tears wrapped up in a startup for it to succeed otherwise.

You seem to be saying the he's very talented, so it's likely not a lack of ability, but a lack of will. Either he's not cut out for a startup, or there are blocks in his way preventing him from executing. Recognize that developers are very bad at estimates and deadlines until they've had years of experience in failing at them. Work with him to figure out why he's missing deadlines (feature creep, unexpected maintenance, infrastructure problems, people problems, misestimation, whatever) and figure out how to help fix those problems, rather than just hand-waving him away as "missing deadlines". Give him the leeway to say "This will take 12 weeks" if it's going to take 12 weeks. Don't try to pressure him into making it happen in 4. What'll happen is that in 4 weeks, you'll have a shoddy, half-done milestone that is going to take twice as much work on the back nine to fix. If he feels like he's being asked to do too much in too little time, that's going to crush his resolve. Estimates should be just that - estimates - and it's critical to not just figure out when a deadline is due, but what the path to getting there is, as well. Have progress indicators to check up on periodically. Adjust the estimate if necessary. Find out what other job-related responsibilities are bleeding off time, and either adjust the estimate to compensate, or offload them to someone else.

You have two paths to take here: Figure out how you can repair your working relationship and restore his passion for the company, or figure out how to get out.

If you can't restore his passion for the company, either due to his being too distracted with a "partyboy lifestyle" or due to that bridge already being burnt, one of you needs to exit, immediately. There could very well be the possibility that his job is hell due to a breakdown in communications and the friction that results from it, and he's medicating that away with drugs and alcohol. That's your one chance to save this - if that's the case, then fix the problems he's medicating away, and you can get it back on track. If not, it's time to start executing an exit.