Income from a retirement plan is income to the recipient. Income to the estate of your son must be reported on the estate's income tax return. Income paid directly to you as a designated beneficiary of your son's retirement plans is income to you personally and taxable to you personally. If you are also the beneficiary of your son's estate, the income to the estate can be passed through to you on a Schedule K-1 (Form 1041) to you to be taxable to you instead of having this amount being taxable to the estate.
If your son died in 2016, you are also responsible for filing his year of death personal income tax return for income he received prior to death, reported to him as recipient.
See IRA Pub 559, Survivors, Executors and Administrators:
https://www.irs.gov/pub/irs-pdf/p559.pdf
Also, the instructions from Form 1041 which includes the instructions for preparing Schedules B and K-1:
https://www.irs.gov/pub/irs-pdf/i1041.pdf
