Our children don't always make the choices we wish they would; they may even go in radically different directions from what we tried to teach them. But as they grow older, we usually lose our control over them, and a wise parent will accept this reality. The best we can do is to try to influence them and encourage them to do what is right.
Your son knows you disagree with what he has done. But he is still your son - and this child will still be your grandchild. Is it best for your grandchild to grow up without knowing you? Is it best for your grandchild to grow up without seeing your example as husband and wife, or learning from you about Christ's love for him or her? No, of course not. The Bible says, "Hatred stirs up dissension, but love covers over all wrongs" (Proverbs 10:12).
Remember too that in time your son may realize the seriousness of his new responsibilities as a parent - and this may lead him to marriage. Pray that God will use this little child to show your son and his girlfriend their need to take that step of commitment.
Pray most of all that your son will realize his need of Christ. Yes, he needs Christ's forgiveness, but he also needs a solid foundation for his life - a foundation that can only come from a living faith in Christ.