Boss says, “Don’t hitch your cart to my horse.” So, now what?

Question: “I am the type of Admin who wants to find a great boss/career to stick with by following the boss as he gets promoted. I thought I had finally found a boss to lead my career path. We had a very good relationship in the beginning, and I enjoy working with him. But when I mentioned I wanted to follow him as he gets promoted, his response was, “Don’t hitch your cart to my horse!” So now I wonder if he even wants me as his assistant. He constantly tells me to keep my eye out for something better and don’t let him stop me from moving forward. I have received multiple ‘above average’ evaluations, so it’s not anything I’m doing wrong. Others say he’s the type of manager who looks out only for himself. How should I handle this? Should I start looking for something else in hopes of landing the right, understanding boss? I don’t want to work with a new boss all over again. I’m ready to settle down.  –Following the wrong lead