You move for YOU and the kids. If he wants to move in there upstairs... that's his problem. Your only responsibility is to tell him why you want to move to that apartment. i.e. the repairs and current living conditions.
You don't mention the other apartment to him. Tell him you want to move there, ask him to come with you and check the place out. If he has half a brain and is so inclined he will see the other apartment on his own.
