Exactly. There's often no release and no transfer. It's especially obvious when he's transitioning from a L footer to a R footer.
At the end of L footers he doesn't release the L ski and transfer balance to the R ski. Since the (still partially weighted) L ski is starting to flatten,it diverges downhill while the now partially weighted (but still tipped) R ski continues across the hill. This widens his stance to shoulder width or beyond and puts pressure on both BTEs. From that "railroad tracks" stance it's impossible to tip quickly enough for a SL turn, so he rotates (ie, pushes the tails).
So... his SMIM is
free foot management with LTE balance during the transition. A great drill for this is the Super Phantom with Touch-Tilt, as described by Max_501:
As in a regular super phantom, transfer balance to LTE of the uphill ski. Then, touch the inside edge of the lifted, downhill ski to the inside ankle rivet of the stance boot ("inside foot arch touches outside foot ankle"). Keep it touching while tipping the free foot further toward its LTE. Don't let that free ski touch the snow until the very end of the turn. VERY IMPORTANT STEP! At the end of the turn, when the free foot touches the snow on its LTE, immediately pick up the new free foot, and touch-tilt the new stance boot.
When learning, you can begin with keeping the tip of the free ski on the snow, but the goal is to keep the whole ski lifted throughout the turn which is a true test of your ability to balance on the outside ski.
To improve balance he should practice gliding on the uphill LTE for a long time. Start as described above but glide on the uphill LTE for a s-l-o-w count of 1-2-3 before allowing the uphill ski to roll over onto its BTE.
Do this drill for many turns... many runs... many ski days. This is not easy to learn. Get video of this drill and post it.
+++
A related issue is his R hand, which often drops behind his hip. This drags his shoulders around to the R, partially unwinding CA, which undermines his balance during the transition. Good drills: Angry Mother and No Swing Pole Plant
***
Running gates or skiing difficult terrain will only reinforce these old movements. We've said it before... get him out of the gates and on easy slopes to learn correct balance and movements.
***
I am not a coach or a PMTS expert. If a more qualified PMTS skier has different ideas, they're probably right.