Because he is feeling trapped and wants no part of this "baby bs".
Getting mentally prepared to leave.
Because that's natural reaction unfortunately, humans are naturally monogamous. Once their job is done something in their brain changes. Not with all, but with a good percentage.
Is he committed to staying with you and being an active, present father in the baby's life? Because while on the one hand it sounds like maybe he's just having a moment of nervousness as reality is sinking in...there's also the other concern which is that he's distancing himself so he can start backing out of that responsibility all together.<br />
I'd recommend having a talk with him sooner rather than later about what might be going on with him and voicing your concerns.
ask him..if he's gonna be a dad he needs to be a man first...confront him..don;t let him act a fool
trying to avoid responsibilities maybe.
maybe he cant believe the responsibility or thinks he isent he father