I am so sorry that you are on your own as this situation has reached a creschendo.
I do think that this is a time to put all your cards on the table hun. If your son has a GF he now knows that sometimes relationships are complex things. I do believe that this is a good opp. to tell it like it is.
He will need some time to take the facts on board, and have a think, who wouldn't! I don't think that he will hate you forever etc...
UNLESS you now choose to compound an oversight with a lie! It's not true etc etc, the other person is lying. Say God forbid that there was a medical emergency and some clever clogs said that it was not possible for this or that person to be your true father - or the only suitable donor for something was from the other family and so on and so forth ... stuff like this happens all the time! The pooh hit the fan at our old neighbours over brother - sister issues but with mid teenagers , it was a hellish couple of months but they are all back to being one big happy family now. ( New grandchild needed family compatability testing for medical purpose.)
I feel for some people in my own extended family who are going to have their whole cosy existance shattered when their Fathers will is read and it proves he has two children from previous liason (who have lodged interest on his estate because he has ignored them their whole lives ). I think that is gonna be nasty ,and is going to cause terrific trouble for his wife who has colluded in the active deception of these sisters.
I would rather be told now if I was your son! I hope this all works out T. ,please give us a shout - if you need to - in the absence of your other half, love SenseX.