If a guy your seeing says he REALLY likes you but isnt sure if he wants a family (because you have kids), how long before you should make him choose to stay or walk away?

Ive been talking to an old high school friend for a few months and about 2 weeks aho we start being excluskive but not "serious". I want to find someone to be in a serious relationship with eventually and i feel that his hesitation toward being part of a family is going to end up hurting me if i stock it or to long to wait for an answer. So id like to give him am ultimatum but i don't know when the best time would be to do it because we haven't been together that long, but when we are we act just like a couple who's been dating a long time.