There isn’t a simple answer to your question. You didn’t mention your other child who will also be affected by this visit. Your ex-husband may not have wanted the baby but he is still the father. Your ex doesn’t seem to be thinking very clearly about the children’s needs. To them, he is a perfect stranger. They are not likely to respond warmly to him at first – which he may take as rejection. If his father doesn’t intend to keep up regular contact, your son is likely to be the one to feel rejected for yet a second time. Meanwhile, the younger child will have to live with his father’s lack of interest from the start.
I don’t think you have enough of the information you need to make decisions. I suggest that you have several conversations on the phone with this man to try to find out why he wants to visit now and what his intentions are for a relationship in the future. Do talk to a lawyer about whether he has any legal right to see the children. Make certain you feel that the children are going to be safe with him, both physically and emotionally. By all means, stay with your son during the visits until you are sure that your ex has his best interests in mind and that he knows how to care for a four-year-old. You also need to know in advance whether the new wife is coming and what they think your children’s relationship with her should be. Once you have more information, I think you will be able to decide how to handle the situation. If not, do consider talking to a family or child therapist. A professional in your own country and culture is better able than I am to advise you in a matter that has potentially important consequences for all of you.
I wish you well.