Please help! I’m having a terrible time interpreting the rules for required distributions when a trust inherits an IRA. What is the RMD & distribution rules for this situation?

Here’s the situation: 

Client set up trust for son – son is neither disabled nor chronically ill. Trust inherited IRA funds from Grandma.

Trust: irrevocable, has it’s own EIN, has the right to continue to hold assets as an investment and does not require funds to be distributed immediately (accumulation trust), mom and dad are trustees.

All other IRA beneficiaries are individuals with no trust.

IRA owner died in 2023 and was older than RBD.