He actually has two problems, I suspect–and I hope you both can face them and deal with them honestly. If you don't, I see little hope for your long-term relationship.
His first problem is commitment. Marriage involves many things, but one of the most important is commitment–a commitment to love each other and be faithful to each other no matter what happens. This isn't just a legal commitment (although it is that); it is a moral and spiritual commitment, as well. Your boyfriend is apparently unwilling to make this kind of commitment.
And frankly, he probably can't see any reason to make it. From his standpoint, he already has everything he wants in a relationship; why (he's probably thinking) should he encumber himself with marriage? If he gets tired of this relationship, all he has to do is walk away — and it happens all the time.
This brings me to the second problem he has–and one you have, as well: Neither of you has given any thought to God and His will for your lives. It is not His will for you to live in this kind of relationship. The Bible is clear: "The body is not meant for sexual immorality, but for the Lord" (1 Corinthians 6:13). My prayer is that you both will turn to Christ for the forgiveness you need, and then commit yourselves to do His will in the future. God loves you, and His way is always best.