Your son is showing just how immature he is. It won’t help for you to give in to this emotional blackmail. In fact, it will confirm for him that he is right. He’s not. He’s on the edge of leaving home and starting a life of his own. He would be outraged if you told him whom he can choose for a mate. You have a right to choose your partner and your happiness as well.
I suggest you tell him as calmly and lovingly as you can that you understand that he wishes that you and your ex hadn’t divorced. You can validate his feelings about not wanting you to move on. Reassure him that you will love him forever and won’t “divorce” him, no matter how difficult he is. But – tell him that you have a right to love and partnership in your life, just as he does. Let him know it will be a shame if he cuts himself off from you for a time while he gets used to the idea but that you will always have your door open for him and that your heart will be with him forever.
Don’t fight about it. Don’t argue. Arguing gives someone the impression that there’s something to argue about. Just stay calm and keep repeating that you love him and will miss him but it’s his choice now what he’ll do. Then hang in there. He’s likely to have a 20-year-old version of a tantrum. Stick to your plans and have faith that the love and care you’ve put into him will come back around. Just be sure to give him a way to come back without feeling like he’s giving in so his pride doesn’t get in the way of a reunion later.
I wish you well.