I'd hate to think the system would let him back up in the sky in his
current state.

--

The "system" is between him and the CFI that signs his FR. Granted, anything
happen and the FSDO just might be knocking on the CFI's door.

The other part of the "system" is the Flight School / rental operations
insurance requirements. If he is ME only, then many may require "5hrs in
type" check out and many rentals require an instrument ticket to rent ME
aircraft.

If he is ME only and wants to fly SE, then he'll need instruction and have
to pass a check ride just like you did, with the full oral exam.
He could take the check ride at the Pvt level, and his certificate would
read. Commercial Pilot Airplane with ME, Instrument ratings. Private
Privileges only Airplane SE.

Oh.. not only will he need a Flight Review, but if he wants to fly on
instruments he will need an IPC. Instrument Proficiency Check signed off by
a CFII.

BT