Under no circumstances, Lie to him, as suggested, you can't trust a liar, ever. Spend your times alone with him, go gently, time is on your side. Introduce your lady friend, if you are in a stable relationship, otherwise don't. In time he will accept someone you are serious with and she will be part of your lives, not the other way round. That wonderful bond you have is precious and will last him all of his life and
your lady friend would want the best for you and if handled with sensitivity, she could be very important to your son too. He has to trust you, like he does, to lie to manipulate a situation would result in him questioning whether anything you say is true, but I am sure you have too much integrity fir that anyway. He will be fine, you have his best interests. That precious bond you have is worth more than gold.