You say she knows everyone at the wedding - how is this, if she hasn't been involved in the family?
My DP is also divorced from his (grown up) DCs' mother. We went to his DS' wedding a couple of years ago. DP's ex didn't speak to either of us for the entire weekend. It was embarrassing. She had never even met me before (because the DCs are grown up, there's no need to, and she wouldn't want to anyway, clearly!). I was not OW - they had been divorced for years when I met him and she's been remarried for quite a few as well. The bride's parents were also divorced, and refused to sit on the same table. I felt really sad for both the bride and groom. DP made it very clear to them that he didn't mind where he sat. In the end, none of the parents sat at top table.
At one point, one of DP's other DCs said to me that he really wished his parents could just put aside their feelings, just for one weekend, just so that his brother could enjoy his wedding without worrying about anyone kicking off.
I am divorced from my DCs' father. My ex's wife was the OW, in a very nasty and sudden breakup. However, I have put my feelings to the side when I deal with ex and her. We communicate, and I actually quite like her now. I would not put either of my DCs in the position of effectively choosing between me or their dad. I would expect them to seat either me and their dad at top table, or me + DP and ex + his DW at top table, or none of us at top table. Not to leave one person out.
Whether your DS' father has been involved daily in bringing him up or not is kind of by the by in this situation, IMO. He is his father. Please don't make your son choose.