I am sorry you are going through this. Divorce is hard enough without the additional confusion of dealing with what must feel like an additional level of betrayal. I think it is important to teach your son about homosexuality and help him understand that few people "turn gay" at some point in their life. I can't imagine how difficult it is, but try to find some compassion for your ex who was probably conflicted by his decision to come out for years. This is definitely a situaton where your son would benefit from professional counseling. Finding age appropriate books about different kinds of families may be helpful as well.

Well first of all i think its awesome you think about your son first. You must be a remarkable person and selfless parent where others would have internalized and I wanted to get that in first.
Second, your ex didnt turn gay, he was always gay he just found the courage but thats another issue. Im thinking your son has no siblings? hes about 12?and that in itself must be tough at an age hes just coming into his own. Is the dad still in his life?
People can be tough and others ignorance can really hit hard. Other children don't get it and will be immature without knowing. I believe knowledge is power and being more informed about his dads situation will help your child. Hopefully your son will learn his Dads decision had nothing to do with him, that his dad was fighting a war inside and it was inevitable whether or not he was his dad. I suspect your child will have a few character building years with peers, teach your child to give in the face of adversity. Openness and honesty will be ok with the right attitude. Find new healthy distractions where your son will grow, such as sports, hobbies, interests,.... show your son that no matter what people do or say, it doesnt change the fact he is a beautiful wonderful person with things to offer others.