i feel like you're getting a bit of a hard time here OP!
i understand your feelings (and those of your son).
BUT
it is inappropriate to consider options that limit the practical and emotional support the mother to be has access to - especially seeing as she's not in a relationship with your son.
it is absolutely appropriate to consider what you and your son can do to provide the most practical, financial and (maybe) emotional support possible, while at the same establishing how much support the mother to be actually wants.
the reality is, this is probably going to be a lot less than your son really wants, at least to begin with.
if he's genuinely serious, he needs to work as hard as possible to be a consistent positive in both the mother to be, and the child's lives.
it's probably going to feel unfair and one-sided a lot of the time.
but that's his (and your) best chance of having a good relationship with the child long-term.
if he can manage this for probably 2-3 years, then he'll be in a much stronger position legally, morally, and most importantly with the mother, and should be able to really start to build his relationship with his child properly.
good luck to you both!