im 19 and have a 5 month old boy.
i was fortunate where my boyfriend stayed with me, supported me and is still with me now.
but in your situation right now, its great if he wants to be apart of your life and the baby's, but put your foot down. and say if he wants to be a part of this life that is going to enter the world, he has to man up. not drop out last minute and leave you hanging.
give him one more chance, and if he lets you down just say you are tired of his endless lies and will not tolerate it any longer. give him a wake up call, dont let him get away with this.
if he wants to be a part of this, he has to prove to you he does and is not going to be a little kid.
you have one child on the way, you do not need to deal with a young mans childish ways too.
good luck hun.