MD,
I really sympathise with you - your dilemma, is, indeed a major one.
Having said that - I believe this whole situation can be simplified with one question i.e. What is the worst thing that could happen if DSS came to the wedding?
It sounds like your main worry is a "major meltdown" biting/kicking/screaming and that your DP would need to deal with this, thereby sacrificing the enjoyment of his/your day and that you would not enjoy your lovely wedding.
There are two simple solutions:
1: exclude DSS
2: invite him and his mum
I can see both sides of the story here - I totally understand your DF's need to satisfy all parties. Poor man is torn in even more directions than you are. I can say as a mother that my childrens' needs always come first though, so try not to be too hard on him for not being stronger with his ex/MIL and his need to be seen as a good dad (it's natural for most parents).
So if you go with option one - be prepared to "force" DP into making the decision and enforcing it - this may lead to problems and imo is not a great way to start married life.
Personally, I would go with option two. There is absolutely nothing wrong with your DP sitting down with his ex and explaining that it's his & your day and that whilst DSS is always welcome, it is one day where he cannot take exclusive responsibility for their child. FWIW I think that when this little boys mum realises how potentially upsetting it may be for her son, she will naturally put his best interests first and make arrangements to suit.
This only leaves you to accept that your DP's ex will be at your wedding or at least be taking part in some of the celebrations. Not great, admittedly, but you will be naturally bound to her forever because you will be sharing the care of a child with special needs. It sounds as if you are already 200% aware of what this entails so it may be less of an adjustment than you think.
Lastly, don't punish your DP. It sounds like he is doing his utmost to accomodate everybody. This is difficult at the best of times, throw a wedding into the mix and it becomes an almost impossible task.
My longwinded point is that if you take a less complicated view of the situation you may find that the solution is not imposisble to achieve after all.