I would explain to your boss (together or individually) that you're planning on moving on, but that you don't want to see his company go belly-up in the process

I would explain to your boss (together or individually) that you're planning on moving on, but that you don't want to see his company go belly-up in the process. Set out a hard timeline (2-3 months, depending on how long it would take to transfer information, knowledge, etc.), and let him know that that's when you'll be gone. That gives him time to find appropriate replacements, and for you to hand on any business knowledge that it required. I would also make a point of starting to document odd processes in the systems you have built, just so that it's documented. I wouldn't go poaching his clients; it's just bad form. If they come to you on their own, that's one thing, but to tell them that you're moving (and setting up a competing business) is, contract or not, a dick move.