For the time being, let your son adjust to all of these changes before he is to meet his father's new partner.
At this age, your son is may be dealing with multiple issues such as - mistrust, anger, possibly perceiving the new girlfriend as a cause for the separation despite the relationship's problems. He may also feel replaced by her in some way. Now, his father is giving attention and affection to another individual.
It would be all right to speak to your son about his reaction to this situation and his emotions. It is healthier to express himself rather than bottle things up.
Perhaps you can speak to his father about the importance of your son's need to have some time to process all of this as well as get used to the idea. Forcing him to meet her can only escalate negative feelings. He could be anxious that she may judge him.
As far as a time frame that has to pass in order to meet her, each child has his own coping skills. What is comfortable for someone after a few months may still feel uncomfortable to another. How you as his mom react to this would also influence his own judgment. If you're open to the idea and encouraging, he may feel that in meeting her he is not betraying you. If on the other hand you're critical or negative, he could adopt those feelings as his own.
Initially, you could suggest to him and his father that he could show your son a picture of her and perhaps say hi to her on the phone as a way of introducing himself. The first "meeting" does not have to be face to face. Your son could ask her some questions and tell her a bit about himself i.e. what he likes, what he is studying etc. It would be helpful for him to realize that she is just another person like anyone else.
If this is a relatively new relationship for his father, it would be better for him to wait to make any formal introductions before he himself knows where the relationship is going. When the time comes for them to meet, they can do it in a neutral place i.e. restaurant versus his dad's home. Or, even during an outdoor event ex: park.