Can't seem to find a clear answer on this...
Grandmother owns a traditional IRA. She dies. Mother is sole beneficiary. Mother starts taking RMDs at age 70 1/2.
Mom now dies. Son is sole beneficiary. How are the remaining IRA assets treated?
1) Does he have to take it all in the year of death?
2) Can he take it over Mom's life expectancy?
3) Can Son take it over his life expectancy? (I don't think so).
4) Something else?

First, mom had to start taking beneficiary distributions the year after grandmother's death, not when mother reached 70½.

The rule on the death of a beneficiary after the beneficiary has started taking distributions is that the distributions can continue over the remaining life of mother. In other words, the distribution schedule does not change.

However, some custodians will insist on an immediate payout. They can do this, even though the law does not mandate it. If you come across that, try moving it to another custodian prior to distribution.

After distribution, nothing can be done.