If it were me, I would want to get all this out in the open before he moves in. That just presents too many possibilities for things to go upside-down, and then he's stuck looking for a new place to live in the midst of painful feelings.
If you have feelings for him and you're at all interested in a romantic relationship with him, then you'll have to learn to be open & honest at some point. Why not start now?
If you keep it to yourself, you're setting yourself up for heartache every time he goes out with a new girl and puts you in the "no" box.
From my impression of the situation, it doesn't sound like there's a lot of chance for this to work out the way you'd like. You've already decided that if you are in a relationship with him, he will be secondary. If he already said he doesn't like to share, I'd wager he likes it even less when he's in a prescribed secondary person. "I love you, but no matter what you do or how our relationship grows, you will never be as important as this other person." Ouch.
Gralson = my husband
Auto = my girlfriend