Hello and thank you for using Just Answer,
Please accept my condolences on losing your son. The taxable amount will be whatever amount in the IRA your son did not pay tax on already. If he claimed a deduction on hos contributions each year on his tax return then the taxable amount will be the same as the total distribution.
The Code T means this was a Roth IRA.
If your son met the 5 year holding period the taxable amount will be $0
You will need to know when he opened the Roth IRA.
Thank you, XXXXX XXXXX had this IRA well over 5 years. That is probably why the box 2a was left blank on the 1099R