I feel like he's keeping me around as a back-up plan or something.
You would feel that way because you are.
I suggest you break up outright and you both explore freely rather than be stuck hoping while you screw one another and others.
Either that or you have a conversation with him about exactly why he wants to keep you around while not being committed so that you know without a doubt where you stand and what his intentions are. Only he can tell you and hopefully he won't lie to you. If he lies, his actions will suss that out soon enough.
“The willingness to accept responsibility for one’s own life is the source from which self-respect springs.” ~Joan Didion