The important thing is that he decides for himself what he will do...Stay a JW or not...The fact that he is dating a catholic is an indication that he doesn't put as much stock in the religion that he is supposed to. But if he decides to stay a witness, I wouldn't marry him unless you and he have an understanding that you will not force your beleifs on one another. That prolly won't work because you seem to want him out. Expressing this to him will more likely than not push him back in because this is what the society tells him to expect from worldly people.
Basically, let him decide first...and then address the issue of whether or not to be married. If he stays with it, the marriage will have significant friction that most people find too dificult to live with. In this case, I'd advise against it.
Quick question, though....If you don't want your BF to be a JW, then why did you start dating him?