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If it is in the divorce papers, that is a court order and it must be paid or he will be in contempt of that court order.
Depending on what the court order states, the spousal support may be non-modifiable or it may be modifiable if there is a change in circumstance. Generally, if it does not specify that there was a waiver of modification, it can be modified upon filing a motion to modify spousal support supported by an affidavit and any documents that indicate why it must be modified. His income, of course is critical to the analysis. So too is the income of the other party. Equally as important is whether either party has had increased or decreased living expenses.
Whether the other parent is reporting the maintenance as income largely irrelevant.
A remarriage could actually be considered a decrease in living expenses since it may be presumed that there are now two people contributing to the household expenses> That would be an argument the other side would undoubtedly make. You, of course, can counter if that is not true.
Your spouse has no legal obligation to support step children. Although you can raise the issue, it is not likely to be given much weight for that reason and because the new relationship is considered a choice that is made with knowledge of the existing support obligation.
I think the critical issue here is the reduce income and the other parties presumed diminished need based on receipt of substantial public assistance.