An Endorsement is not a Rating
Do an hour of ground about tail wheel aircraft procedures. Do a couple hours in the pattern until proficient. Sign off the tailwheel endorsement. Sign off the flight review.
Or, document appropriate tail wheel training in FAA WINGS, he does a couple online courses and automagically gets a flight review in the background. Even if he's not yet skilled/proficient enough to gain the endorsement.
Nothing prohibits learning something new as part of a flight review.
Terry