Bells, it sounds like you are being practical and thoughtful about it....communication is important and observing behavior. The amount of time a person decides to devote to their relationship(s) depends on how important relationships are to them.
There are plenty of people who want to have a relationship but they areen't interested in spending time nurturning it.
I have always had multiple emotionally intimate relationships- even before I was poly......even when I was in elementary school....through high school and ever since then. There have been many times when people have said to me "I don't know how you can juggle so many relationships"....but it's not juggling to me. It's who I am. Most of what I value in life can be experienced through intimate relationsips- therefore I like to have a variety of them and I alway have.
So..... being poly might not have a lot to do with it in his case. The question might be.....is he one of those people who has always had a variety of emotionally intimate relationships that he likes to "juggle" because that's just who he is??