I completely understand you and I would be scared to that the T would leave. As he most likely will take some time off to be with his child, even if it´s most often during a shorter time than for a female T having a child, I would bring the question up more or less directly. If I wasn´t able to ask him in a session, I would e-mail or text him and explain that him getting a child makes me upset and worried about how therapy will look like, time off and so on.

I don´t know for how long you´ve seen this T but if you think you could and want to find a new T, I would consider looking for a new T if my T told me he would be away for several months and perhaps during several periods of time to take care of his child.