I thinkk you are feeling a bit sensitive at the moment.
Firstly do not blame your husband in any way or feel that he has failed you. Do not let it cast a shadow over your strong decades-long relationship. It sounds as though he knew that this would hit you hard and it was good of him to try and think about ways to discuss it with you that would reduce your hurt. He used a relative as a sounding-board as to how to do the right thing - that is fine.
Secondly, every child is a gift. I know this is a cliche, but it is true. When a GC arrives, under whatever circumstances, we love them and do all we can to help in their upbringing. If this boy proves to be your son's I am sure that you will be there for all of them.
Thirdly - do please stop blaming yourself and questioning how you brought your son up. I have to say that I think you have every reason to be proud of him in the way he is responding to this situation. He sounds as though he is a decent honest young man seeking to do the right thing. OK he has made a mistake - who of us can say that we made none in our youth? - but he is responding honourably and he is a credit to you.
Fourthly - young men get randy and being on holiday lowers the defences, especially if a bit of alcohol is involved. He is only human; and I am sure that he has learned his lesson and taking all steps to cover the STD concern.
Please stop beating yourself up!! - hold your head up high and give him all the support you can.
BBC Two- Surgeons: At the edge of Life
How do you acknowledge Easter.