If his dad is interested in having a relationship with his son, is not abusive , then this has a potential for positive outcome.
Your son will largely respond based on the vibes he is getting from you. You can prepare him about the possible reunification. If he had asked about his dad in the past, shown interest/curiosity about wanting to know his father and to have some sort of interaction with him, then your son can benefit from this. You would have to explain to your son objectively what had taken place in his dad's life, the fact that people do not always do what other's expect them to do, and that people learn about each other with time. Focus on some of the positive aspects - the two will learn about one another, will be able to share things/activities, he can be a bigger brother to his half sibling, etc.
Whether or not your son meets his dad won't change the fact that this man is his father. For whatever reason he had been absent from your son's life but now it seems he wants to enter it. If you believe that your son will not be abused in any way- emotional, physical/psychological, then keeping in touch with his father opens the door for some sort of future the two can share.
Your son will get cues from you, his father and step mother about how to respond in many of the situations that are to come. Remaining non judgemental and open minded and positive would be helpful for him with the transition.