It states very specifically in the memo he refers to:
"It is recommended (not required) that passengers bring along any supporting documentation (ID cards, letter from doctor, etc.) regarding their medical needs."
So yeah, it would likely be easier for him if he did have one, but it's certainly not something the TSA can demand.
They can't demand it, but they also aren't obligated to let him through unless it is evident that he meets the condition for the exemption. If it's not evident that he's eligible, how can it come to pass?
"It is recommended (not required) that passengers bring along any supporting documentation (ID cards, letter from doctor, etc.) regarding their medical needs."
So yeah, it would likely be easier for him if he did have one, but it's certainly not something the TSA can demand.