Dear Agnostic, 

A most unusual and incredibly perceptive and generous question that brings up some very important points. 

You say you can't imagine actually marrying the young man you are dating, so it seems that what you are looking for is merely friendship. At the same time you don't want to be hurt, which means you have the suspicion that there is some attachment there already or there is the possibility that it will arise. That is the nature of the almost exclusive friendship that dating means. You like each other, you are comfortable with each other, you spend time together, enjoy doing things together, you begin to open up to each other, trust each other... That is the natural, progressive dynamic of a relationship. If at any time you get uncomfortable with each other or don't like the way things are developing, generally you break up and go back to being "just friends", breaking off the dating progression. 

This is why you are wondering if you should perhaps not be dating. If he feels he is called to be a Jesuit but the two of you date, there is going to be a "showdown" at some point in time, and he is either going to walk out on his conscience or walk out on you, something that will be difficult and hurtful for you both if your friendship and attachment continue to deepen through your dating. 

I think that the one who has to make up his mind is your friend. If he thinks he has a vocation he should make a decision and not mope about. If he thinks he has a vocation he should not date in order to test his call and in order to spend some more time in prayer. He does not have to turn into a recluse, but he should limit his dealings with women to normal, non-exclusive friendships (in other words, dating will not help him come to grips with his call). 

Hope this helps. God bless (He does exist!).