If you're planning to keep the baby it sounds like he's at least interested in a coparenting relationship. Maybe ask him if he'd consent to going into some family counseling with you to discuss how to make this work best for the child. You could then bring up things like living together as a family and committing to each other. He's 51 so it's probably not like he's out there chasing other people (or at least not having a lot of success with it). So this may be the result of him having had a bad past relationship, being embarrassed about the vast age difference between you or some other thing that can be resolved with a (therapist guided) conversation.
Maybe stop spawning children in the hopes of making your partners stay with you.
If you're planning to keep the baby it sounds like he's at least interested in a coparenting relationship. Maybe ask him if he'd consent to going into some family counseling with you to discuss how to make this work best for the child. You could then bring up things like living together as a family and committing to each other. He's 51 so it's probably not like he's out there chasing other people (or at least not having a lot of success with it). So this may be the result of him having had a bad past relationship, being embarrassed about the vast age difference between you or some other thing that can be resolved with a (therapist guided) conversation.