All you really have to say to your son is that grownup stuff is complicated and it's personal and private between you and his dad. Assure him that you both love him and that you will always, always take care of him.
Don't expect him to feel great about it, but he WILL adjust. My own adult kids have told me what confused them the most was that their dad and I never fought. I left their dad when he was sober like 15 years, and we had a very amicable divorce. FWIW, my kids are happy and healthy and had a pretty great childhood. My older son had some issues in adolescence that were pre-existing--we saw signs way before we separated.