This is one of the hard things about being the mother of sons. The girls have all the "choices". As the parent of a dtr if she gets pregnant...well you know its your grandchild! For a parent of a boy, you may have to wait 9 months and then longer for DNA to come thru to tell you if it is your grand child.
I can only imagine how hard this is for you but there is nothing you can do at this point. Well you probably could but it would cost a ton of money and your difficult child would have to hire a lawyer and claim he didnt know he was a father and just found out and was denied the right to sign away his rights...blah blah. Then would you want to uproot a child who is now securely placed with a family where he has bonded? Would you want to risk the upheaval that your son could bring into this childs life?
If it were me, I would maybe attempt to find out the birthday of the child and do something quiet for myself every year if that would make you feel better. Im sure you will have more grandkids in time.
I dont know if I made a lick of sense...lol.