My boyfriend's mood swings about our relationship..

This is the telling bit. Just think about it. Is this really someone you truly want to be your life partner?

But in all honestly I think he's missing his freedom. What was once a bit of fun turned into an inescapable constriction. You say he doesn't know what he wants, but I think he does. The reason why he probably feels down is because he doesn't want to hurt your feelings by saying that he's not ready for a relationship. Being in a relationship takes commitment and maturity. Most people gain maturity in time which is why people in their late 20s and onwards are more inclined to 'settle down'. I just don't think he's ready for a relationship yet.