I'm not going to buy my significant other the kind of proposal ring they want
Why not? Can you not afford the ring?
No, it's because I'm not going to marry him and he's not moving to California with me.
That seems fine, but make sure he knows and don't lead him on. If you aren't marrying him, you shouldn't buy the ring.
Okay, I'll let him know.
It's good that you're going to be honest, because it's better for everyone in this situation. You'll both be able to move on.