Forgive me if I repeat anything others have said - I have a headache and didn't read through all the replies you've already received. But I did want to mention a couple of things.
Unfortunately, this is going to complicate things for you guys because CIC will not know how to deal with it. In fact, they will probably delay finalizing the application until after the birth of the baby so they can compel the child's examination, but don't be tempted to avoid disclosing the impending birth so that it isn't an issue for this application because later, when he tries to sponsor his child and his/her mother, CIC will try to make her and the baby inadmissible because he didn't disclose his impending fatherhood before he landed.
If your son marries his gf now, he will no longer qualify as a dependent child and will not be able to be included on your application. In addition, prepare yourself for CIC to make an issue of his eligibility as far as whether or not he is in a common-law relationship with the mother of his child. If they can make a case in that regard, they will find him ineligible that way. This is not going to be a simple situation to deal with - even if it seems clear cut now. Hopefully they have not lived together - be prepared to prove it - and once you all land in Canada, he can marry her (assuming that's his intention) and sponsor her to come to Canada, too.
In that regard, whether the child is born before or after you submit the ap, designate him/her as a non-accompanying dependent - that allows him/her to be included later on the sponsorship ap with his mother. Also, be aware that sponsoring a dependent child with a child of his own means that you will have to meet minimum income requirements to be eligible to sponsor, even though the child will not be coming to Canada with all of you and will actually be sponsored later by your son. He will not have to meet minimum income requirements, though.