I can see where you're coming from OP.
You're trying to do the best for you're child and see a huge imbalance between how your DN and DS (and also BIL and DH) have been treated.
I know a financial contribution would be a benefit to your DS, but given the toxic nature of your in laws, the cost would be akin to taking money from a loan shark - the re-payment being far in excess of what was provided (emotionally and demandingly as opposed to financially).
My PIL's are committed Catholic's. They are lovely people and can't do enough for their GC.
However they know people (through church) with similar attitudes to your PIL's.
They are vocal about tithing significant amounts plus annual "show" donations to the church for upkeep for specific items (usually those that have a brass plaque with their names on it).
Yet their children and GC get very little "investment" (financial or emotional) whatsoever unless they "conform" by attending church, praying etc etc.
PIL's can't understand it. It seems (and imho is although I'm not religious) very unchristian to them.
They tithe but have never put any pressure on the rest of the family to "pray" for assistance from them or attend Mass.
They give it freely - because they are genuinely nice people and whilst strong in their faith they don't think you can, or should, "bully" people into finding God (a disingenuous faith based on gain is not true faith).
So no, I wouldn't ask. Not because you have nothing to lose, but rather I think you and your DS have a lot to lose by giving them some leverage over your lives.
Your DS is far more likely to flourish with your unencumbered emotional and practical support than with the baggage any financial assistance your PIL can give.