Your issue isn't how to bring this up to him, your issue is that you're treating this like something personal when it isn't.
People leave jobs for all sorts of reasons. If management isn't prepared for that, then they're really shitty management, no matter how well they treat their staff. Being prepared for improtant staff moving in is a critical part of running a company.
How do you bring it up? You do it in writing and keep it simple. When your boss follows up with you, you can choose to explain if you want to, but you don't have to. If you tell them it's because you want to move away, then even the most poorly prepared and unprofessional manager should accept that graciously.
But really, as I said, the bigger issue is why you are internally struggling with this. That's not a healthy relationship to have with an employer. It's fine to feel grateful, and gracious towards your employer when giving notice, and to do what you can to ensure as smooth a departure as possible if you want to, but in no way should it cause you angst to move on from a company if you are ready to move on.
It's not personal, it's business.