I understand you want your son to be involved and supportive of this pregnancy and adoption but, that may not be what he wants. Some guys are unsupportive or uninvolved throughout the adoption process. Other times, withdrawing is how men handle the feelings that come with the choice of adoption for their child.
Adoption is a complicated process emotionally when it is a choice made by the birth parents. Your son may wish things were different, or may wish he didn’t have to make this choice. His ex-girlfriend may be pushing him away or assuring him that she will take care of things.
Legally, his rights are the same as hers, however he does not have to be as actively involved since he is not the one that is pregnant. Most states allow men to sign the papers, or take no action after they have been legally served. Similarly, since the relationship has ended, he may feel like he doesn’t have a part in the pregnancy.
You can always reach out to his ex-girlfriend and see if she would be willing to allow you to be part of the adoption process and stay in contact with the child after the adoption, even if your son doesn’t want to have any connection with the child.
Being supportive to your son is the best thing you can do right now. If he is like most guys, he likely won’t open up emotionally and tell you how he really feels, but loving him through this time shows him your support of the decision for adoption.