The code in box 7 is determined by the payer of the 1099-R, not your son. You'll have to use whatever letter/number is in box 7.
He might, though, qualify for the abatement of the penalty. After you enter the 1099-R exactly as you have it in your hand, you'll continue on through the section till you get to the screen below:
The definition of
. If you become disabled before you reach age
591/2, any distributions from your traditional IRA because
of your disability aren't subject to the 10% additional tax.
You are considered disabled if you can furnish proof
that you can't do any substantial gainful activity because
of your physical or mental condition. A physician must determine that your condition can be expected to result in
death or to be of long, continued, and indefinite duration.