If you and your partner of 21 years believe this will be a joyous event, then I would allow your son to join you in this celebration of your love. I am sure that you can tell your story in a manner that is both truthful and sensitive to his feelings. He may become confused upon hearing your revelation. He also might initially question whether there are other lies that he has been told. Neither of these potential responses means that he cannot come to understand and accept the history of your past as a couple and a family. What is most important to him is that he knows that you and his father have loved him, appreciated him, and raised him with great care and affection.
I wouldn't suggest that you marry quietly and ask him to keep it a secret because there is no reason to be ashamed of what you're doing. You might wish to engage the talents of an experienced family therapist to advise your family during this unusual and emotional time. Congratulations on your upcoming marriage and your commitment to being honest with your son.