Question seems to come up every year.  Here's my answer from last year, along with @IRonMaN 's flip remark.  

https://accountants.intuit.com/community/proconnect-tax-discussions/discussion/social-security-benef...

What I'm still not sure about is whether the "income in respect of decedent" is the amount that would have been taxable to the parent, based on the parent's income; or the amount that is taxable to the son based on the son's income.  For example, if the parent had no other income (which could happen with disability benefits), then none of the retroactive payment could be taxable. On the other hand, if the son has $100,000 of income, then 85% of the Social Security could be taxable.  

Does the 1099-SSA also have the name and SSN of the parent somewhere on it?  That might be helpful. Whatever method you choose, I would report it as "other income," not on the line for Social Security benefits, and attach an explanation.  But I would still be prepared for a CP-2000, if using the son's income it comes out as taxable.