Here's my .02.
First, if the first female is the mother of his child, don't even bother with it. Too much drama when you're dating you're best friends ex and there is a child involved. Believe me, I've seen it happen.
Now, if this is not the case, and he is interested in another female and not the first one, then get to steppin'! If he has that type of an attitude, ya snooze, you lose, then you're wasting valuable time. Talk to the chick and see what happens with that. If he does catch feelings about it, just let him know about his own policy. It's not like you're doing it behind his back or something. Clearly you're talking to someone who is available. So do your thang, and hope for the best.
While you're at it, and if it is up your alley, try some of the other apprentices too. Never hurts to have choices.