That's a LOT of social security.... is it for prior years?

And if the dependent received it, under their SSN, then it goes on THEIR return.

If it is for back years, you can 'average' a lump sum distribution (see link to a recent thread below) .... but if it is for the dependent, is any of it actually taxable?

https://proconnect.intuit.com/community/individual/help/entering-a-social-security-lump-sum-payment-...

And if it's all under the mother's SSN, that's called income assignment and it's all the mother's income to report.  You'd then need to determine the nature and character of the transfer from the mother to the son, whether that's a gift or something else (e.g. genuine expenses for her care or other expenses paid on her behalf).