what a difficult situation OP.
I think you need to be careful about not alienating your DS and partner. Mentally compartmentalise the fact they're buying a poor woman's body and commissioning trauma on an infant and just project forwards with the anticipation and excitement about the new baby.
This innocent baby is already at a disadvantage due to the trauma of early separation that it will endure, so it deserves to be brought into a wider family who love and want it (even though of course you wish they hadn't gone ahead but you can't change that).
Whilst there's nowhere near enough support available, children who are adopted at least have their early traumas recognised: their school will be informed and they'll get priority placements for school admissions.
Surrogacy is couched in euphemism and denial (designed to make palatable something we understand as wrong for any other species ). This places children born to surrogate at a further disadvantage as it's probable that their early trauma will not be acknowledged by their caregivers or schools. The commissioning parents are likely to look for any other reason to blame for any future difficulties and society will go along with that as western medicine has a long tradition of blaming the individual ("brain chemistry" or "genetics") and ignoring the huge roles that trauma, socio-economic factors, and early environment play in future health.
You can at least be one adult in this child's life who acknowledges the trauma they experienced when they were most vulnerable and who holds that in mind as they grow up. It might just inform the quality of your interactions together and that could be quite an important thing for the child. I'd suggest researching this area online.
(I'm close to a kid adopted at birth and it has of course affected them in many subtle ways. The understanding they got from their teachers was utterly invaluable. All kids need someone fully in their corner).