As others have said OP, I really think you need to do a lot of self-examination here. He is complying with your dictat and keeping his relationships well away from you. You have got exactly what you asked for. You want something different now, and it appears that the change is for your own selfish wants and needs, and not because you genuinely want to right a wrong.
You actually don’t sound that sorry about what you did to your son, nor regret the hurt caused to him- not truly- you appear to be most upset that you cannot get what you want now- which is access to your son and his partner because you think there is a possibility that this could be more serious than previous GF’s and you hope more grandchildren could be on the horizon. In other words, you want to “fix things” now because there is a benefit to you- access to potential future grandchildren- not because you want to make things right with your son for his own sake and because you miss having a full relationship with him. If you did, you would have tried to fix things a long time ago and not only when you sense there may be more grandchildren in the future. I suspect this is likely to actually deepen the hurt he has- “I’m still not enough, she only wants to bring me back into the family now that I am keeping up appearances and because she wants to see any children we might have”. If you want to fix this then you really need to fix yourself first.
I think he will have picked up on your actual motives for welcoming him back into the family and is keeping you well away from his partner in part because of this. And probably because he does not trust you, so is keeping the important parts of his life well away from you. He is hurt and angry- rightly so- but I would imagine that he also does not trust that you care about him, he knows he is not a priority for you because you’ve made that very clear- you have happily thrown him under the bus, so to speak, by excluding him from your family in order to get what you want (seeing your GC). Why would he want to expose the person he loves to that? He is keeping her away from you for a good reason, and until you really comprehend that and the real impact on him of what was done by you and your family- until you can admit it to yourself and honestly be accountable for your own actions and the consequences for him- I don’t see how any apology can be sincere or meaningful. Apologies which are a means to an end, done to get what you want are neither sincere nor meaningful. It also means that nothing has been learnt and you are more likely to repeat the pattern of behaviour (not necessarily do exactly the same thing, but respond to situations in a similar way), so trust is harder to rebuild between you and the injured party.
I think you may also have to prepare yourself for the possibility your son’s partner knows all about what you did, the impact on your son and how he feels about you and his wider family, and has decided she has little interest in meeting you or building a relationship with you at the moment. Your son may not want to tell you this, either to protect your feelings (the irony that would be) or because it opens a whole can of worms that he prefers not to get into, so he just makes excuses instead. Perhaps they have even decided together that he will have some limited and very structured contact with you to keep you at arms length emotionally, but they will not have “family” contact with you. That may change if you can rebuild a relationship with your son, but it might make her very wary in the future despite this and require a lot of building of trust. She has every reason not to trust you with the wellbeing of any children they might have- you didn’t treat your own son very well, why would she trust you with her child (if they chose to have them)?
You have justified your actions to yourself and painted yourself as a hapless victim of circumstance in your own narrative. You are not- you say you agreed with your daughter’s opinion- which is absolutely daft, if you ask me- and chose to cut him out of family events as a result. You CHOSE that. It wasn’t done against your wishes- you have clearly stated that you agreed with it. You say your DD would not have come to see you and so you chose to prioritise seeing your GC over seeing your son. You chose not to make your son your priority. You chose not to try and find a compromise- invite them to alternate events? Speak to your DD and son about a way forward? No, you aren’t a helpless victim in this situation- you made a series of choices which have caused pain to your son. You perpetuated this over the years and now your relationship is more strained and distanced as a result.
Yet, you seem surprised, when this is an entirely predictable outcome from being ostracised from his own family on the basis he had a few girlfriends and his relationship status did not have the right optics for your very religious DD and SIL. Essentially, you put their desire to appear to be “the right sort of family” above actually being a family. You have made it clear to your son he is not a priority and are upset that he is now acting accordingly? I think it takes a great deal of self-delusion in these circumstance to get to that point and unless or until you can be honest with yourself, you will not be able to address the issue with your son because you won’t be able to truly own what you did, sincerely apologise and hope you can start to rebuild things. If you cannot truly see that the issue is actually about you, your choices and your actions and the fact that even now, you only want to make amends in order to get something from him- getting to meet his partner because you’ve feel if you don’t have this then it will be harder to get access to your GC- not because you want to repair things for his sake, then nothing will change. Your position is so self-centred it is almost breathtaking.
I know this seems harsh, and it is probably hurtful to read, but I am being honest with you about where I think the problems lie. Sugar coating it and sympathising won’t get you what you say you want.
I think if you genuinely want to rebuild a relationship with your son, for his sake then I think you really have to understand why you are in the position you are in and clearly see the role you played. I think you also need to understand what you have done and examine your seeming selfish reasons for wanting things to change. You have to change your position to get any change in position from your son, you need to change things for him to trust that you won’t do something similar again. I think you need to acknowledge that not only was what happened wrong, but that you did wrong- no excuses or trying to wriggle out of responsibility. There may have been reasons behind your choice that were not down to you, but you must own the choices you made and acknowledge the damage caused. If you don’t, the apology is empty. For the apology to be meaningful you also have to understand why you did what you did, and if you do feel it was wrong/have regrets, then you need to work on yourself before diving into trying to fix your relationship with your son.
And if all this really is just about wanting to see any hypothetical GC and not really about wanting to make amends to your son, then I think you should leave them alone as you will only cause more hurt to your son, and are unlikely to get what you want, as I doubt he or his partner will want you to have a close relationship with their DC.