Oh D., believe me when I say I feel your pain. We have a 20 year old who is in a very similar situation. His girlfriend is 28 with 2 kids, an 8 year old boy and an 11 year old girl. She too did not really raise them but now, she and my son are talking about having a child together in a year. I know she wants another baby and I guess our son is the one to give it to her. It makes me sick thinking about it because we all know how this will turn out.
Our son was doing ok, graduated from high school, was working and had a nice little nest egg from living at home. He got involved with a girl we lovingly (haha) call "the Devil" and everything went to heck in a handbasket. He left on very bad terms (the police were called) and we didn't know where he was for a while. He did eventually contact us again, and of course now "the Devil" is long gone (Praise the Lord for that), but so is all that money he had saved and he's since been fired from a job we begged him not to take and he lost all the benefits and his seniority at his other job that he had to go back to in order to pay his bills when he got fired. Believe me. We have tried and tried to tell him what would happen each and every time he's made a bad decision and it doesn't matter what me, my husband, his grandmother, aunts, uncles and family friends tell him, he's going to do what he wants and then when it works out exactly like all of us said it would, he acts so surprised and victimized by the outcome. It's so completely frustrating.
All I can say to you is if you want to keep a relationship with your son, you just have to grin and bear it. We don't like the 28 year old girlfriend now and we especially don't like them talking about having a baby with my son being so young, but what can we do? We are here for him and have helped him a few times out of some financial jams he's gotten himself into. We've helped him when he's had car trouble and we have even been so supportive as to let him bring his girlfriend to our annual Thanksgiving/Christmas celebration with my husband's family in Missouri so that everyone could meet her. That was really hard for me. When he tells us about things that are going on, we give him the best advice we can, but to be honest, he has NEVER made the "right" choice even after being given all the relevant warnings.
Unfortunately, this is not uncommon. It's just the will of the child and after they reach the age of 18, there's nothing a parent can really do but pray for the best. You always hope things will be different and you'll end up with the perfect child who goes to college and gets a degree and puts their career first and doesn't drink or smoke or do drugs or have promiscuous sex and blah, blah, blah, but I've yet to meet many people who have that kid. I've met a few in our shoes though and they always say the same thing. Just pray for them. Be there for them. You don't have to approve, but you have to accept if you want to keep them in your life.
Good luck to you. I know the pain you're in and I'm very sorry you're having to go through this.