Long term relationship, needs space but continues to contact

So my boyfriend and I have been together for 4 years. Long story short, we have had some bumps on the way. We have broken up a few times, seen other people. All in all the break ups don't last, and the other people don't last. We both agree that it is right between us and we love each other more than words can describe, we decided it was time to move forward and get a grasp on a more stable relationship; marriage. (The subject has been a topic of discussion in the past numerous times). But the problem is recently about every other day it is back and forth. He started off with he needed space to figure things out, I agreed with him and let him have the space, I even stated marriage could wait. A few days later he started contacting me again just like nothing was wrong we talk normally, we see each other. A few days later after the greatness he decides he needs space again and he isn't sure of things. But it comes down to not speaking for a day or two and then he contacts me. I need him to be serious and work out any issues he is feeling but I have a hard time not responding when he does contact. I already told him today when he stated he needed space again that this was serious and he shouldn't contact me until he was fully certain on his decision of either walking away or being with me. What should I do or say if he contacts me again and then shuts down a day later??