If you really love this guy and he really loves you and your child then why not get married? You don't have to have some big humongous wedding but going through the process of being engaged, preparing for the wedding, your son participating in the wedding, etc, it would be much easier for your son to understand that he has a new daddy. And, if either of you aren't "ready" to get married, then why would you put your son through the heartache of experiencing a possible breakup? I'm speaking from experience....my stepdaughter has had 2 serious boyfriends that she's lived with (neither of them her son's father, one of them her daughter's father) and I have watched her children become more and more detached from people that care for them. They believe that people that love them, leave them. How tragic!! I'm sure you're anxious to start your life together and you're totally in love...I remember those days. I was 31 when I got married and we got engaged and we were going to have a huge wedding then a couple of days later I decided wanted my life to start with my husband then ---not in a year!!! So, we got married 2 weeks later. (And, I wasn't pregnant---haha--some people thought that's why we got married!)
Your son needs to see a beginning, an embarkation point, that's what a wedding does....moving suitcases and boxes in to your home, that isn't a starting point....can you understand his confusion.
I'm not trying to lecture....I just think everybody thinks today that you have to live together....you don't!! Did you know that twice as many couples breakup that are living together versus people that are married? Something to think about....if you really love this guy, don't you want everything to be exactly right for you and your son? I pray that God will give you patience and wisdom to make the right decision for your family.