It's possible that he's afraid that if you succeed in this audition, you'll leave him behind or out-shine him. Some men don't like it when their partners do better than them. "A woman doing better than a man?! Unthinkable!" Of course, he may not be that much of a dick and may simply think that the more you succeed, the less time you'll have for the relationship. Of course it could also be simple jealousy. If his life is going nowhere and he knows it, he might be envious of the fact that you are making strides to succeed in your own life. Neither reason, however plausible, is justified. If you're in a relationship and you both have dreams and one of you succeeds in making said dreams come true, then be suppprtive of your partner and be there for them. Don't hold them back because you can't move forward. Even if they do leave and don't consider the support you gave them, make them remember that you were there for them.

He's just trying to be nice. He really thinks you stink at your art form, and your audition will be a humiliating failure.