If he is strict (rabbinical) kosher, it may be a moot point, because he won't eat anything you've prepared anyway because your cookware wouldn't be kosher, nor would your plates, glasses or silverware. If you have paperplates & plasticware, he may use those things, but the only food he would eat would be either what he brings to the party himself or what comes directly out of an unopened package with an acceptable hechsher (kosher mark). However, since he is dating your sister and you all are obviously not Jewish, I'm going to say he probably isn't that strict, but your best bet is to ask your sister about his dietary requirements. If you want to keep the birthday dinner a surprise, then just ask as if you are planning on hosting a general party for some other reason (maybe to get to know the new boyfriend?).
at 2:51 PM on Nov. 17, 2009