Your DS wants your Exh sat next to him and not his Stepfather? what idiot came up with that shit? Surely not him all by himself?
For him to even consider this shows a complete lack of thought or gratitude for what your H has done for him and is affording his feckless spermdonor of a dad levels of kudos and recognition that he has absolutely no right to. the fact that he would smugly relish boosting the man who ACTUALLY raised his son off to another table is icing on the proverbial cake.
I want them to have the day they always wanted, just not at the cost of my mh and the relationship with dsf.
I would sit them both down actually and bring them BOTH right up to speed as to what kind of man your DS dad was and how he has NO right to be sat at ANY kind of table tbh, let alone the fucking top one and certainly NOT by YOUR side. Your son is old enough to get married, he's old enough to learn a bit about the kinds of creatures that walk this earth and what they do to others.
I'd also go further to say that you are adult enough to be able to be in the same room with this man who made your life a living hell, for their (warped sense of family) sake, but you will NOT allow him to be elevated to ANY kind of position of authority or standing by being seated next to you.
I'd also point out that YOU were there for your kids, YOU raised them, YOU fed them, YOU kept a roof over their heads. YOU dried their tears, and put plasters on their knees. All this while recovering from a seriously violent and abusive relationship. Then you met a man who took them all into his heart and has been more of a father to them than their own, and to have his love and efforts snubbed in this way is out and out rude.
Suggest that either DH and you sit on the top table and ex sits at the end with the bestman if need be, or the ex can sit on a table close by. He should and will be happy just to be there, he has no automatic right to it.
Your future DIL is 'lucky' in that her family has not been blighted with abuse, but your's has and if anyone knowing the truth of what's gone on at the hands of this 'man' would never in a million years seat him next to his victim and expect her to suck it up.
Time to tell the truth about the ex. Your DS doesn't have the information he needs to make the journey between boy and man. he needs to 'get this'