He must file a return if you file separately and 85% of the SSDI will be taxable ... this is not a choice it is a requirement so filing a joint return is usually best but you can work the returns both ways and see which way is best for you both.
Did you just get married in 2017? Yes you should file a Joint return. You will get a personal exemption for each of you. That's the same as claiming a dependent. A spouse is never a dependent. You just file as Joint which is 1 return combined for the both of you. You will have to enter his SS and SSDI. It may or may not be taxable.
Up to 85% of Social Security becomes taxable when all your other income plus 1/2 your social security, reaches:
Married Filing Jointly: $32,000
Single or head of household: $25,000
Married Filing Separately: 0
And info on Getting Married…..
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Is it better to file Joint or Separately?
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You must file married, either jointly or separately. Joint filing usually has lower tax rates and is allowed even if one spouse does not have taxable income. The only down side would be if your husband owes debts from before your marriage (such as student loans or child support) that might be collected from a joint refund.