Your talk with Jacob might resemble something like this: "When I first met your dad and began to love him he was ... (here you mention the positive things that drew you to him and caused you to fall in love with him, if you did). As we spent more time together, and especially after you were born, I realized that he really wasn't going to be a good father for you or a good husband for me; he just wanted to keep doing things that weren't grown-up. He was a person who just wasn't ready to be a daddy or a husband with a family. I wanted to be a mommy and have a family. For a long time after he left, I felt really sad and lonely and I got very angry with him, like you probably do, for leaving us. But after a while, I knew that you and I were not the reason he left. He left because he couldn't love us and take care of us -- I guess he just wasn't ready to. I thought your dad would make a good father and a good husband, but I made a big mistake. So if I ever love another man, I will make sure that when he loves me, he will also want to love you and be a good daddy to you. That's a promise."
Take your cues from your son as to how much he really needs to know; reassure him in many ways of your love and that you are here to stay; expect him to be angry, sad, confused; and don't try to pretend that you are just fine with all this. This move is part of an ongoing series of events about dealing with feelings that will teach your son that there is pain in life and that in time we will heal. Good luck. I know this little guy will get the best of your love, empathy, and support.