The two people that are in my life right now are very very different from each other - I often say that I don't see why I would want two people that are the same. Each finds a different resonance with parts of my character that I love.
So if I understand this correctly, he isn't interested in having more than you, and you can imagine having more than one relationship.
Here are some things to think about for yourself, in case you haven't already:
* Should the person be male or female?
* Are you expecting them to only have you, or can they have other relationships? if the latter, can they be married?
* Can the person already have children?
* When you picture your desired living conditions, what do you see? Each person living alone, or two folks together and a third separate, or all together?
* What does your old/new boyfriend want/need out of this? Are there certain expectations that he has?
Share this here if you want to - but I think they are a good start to working out what you are looking for, if you aren't sure yet.
"Listen, or your tongue will make you deaf." - Native American Proverb