A prenuptial agreement has no effect on tax liability and filing requirements.
If you will be married - you will have a choice to file either jointly or separate.
If you file jointly - 85% of your fiance's social security disability benefits will be taxable.
But taxes will be calculated based on married tax schedule - that is lower than for singles.
If you provide the amount of benefits - I will estimate your tax liability.
assuming you file single, with adjusted gross income $75,000, standard deduction, no dependents, no other credits or deductions
in 2009 your taxable income would be 65,650 and estimated tax liability 12,600
Your to-be-a-spouse will have $1780 * 12 = $21360 SSD income from which 85% will be taxable - $21360 * 85% = $18156
assuming you will file jointly - that amount will be added to your combine income - total - $93,156
your joint tax liability would be $11,902
So - in your situation it is more beneficially to file jointly.
Let me know if you need any help.
If you are not married - each of you will have a separate tax liability - your estimated tax liability is above and he doesn't require to file.
If you file separately - your tax liability will be the same as for single and he will have additional tax liability - sorry - I do not have a calculator handy to estimate it.
The best way seems - to file jointly - assuming you get married.
But my calculations are based on simplified assumptions.
Your real situation may be a little different.