Does he have a late release? IMO to bring down the ball he needs a stiff tip profile this is the only equipment way he can drop ball flight. KBS Tour 120's are going to be a high ball flight on average. If he goes to a Dynamic shaft tip stiff these can "feel" boardy to those with a medium release who don't compress the ball well especially compared to a KBS Tour 120. It's a Catch-22.
The Apollo CWS stepless .355t which was discontinued but I get them thru Swing Science their branded shaft line works well at a better price. Possibly you might find some around? https://www.apollogolf.com/apollo-golf-shafts/apollo-cws-steel-taper-tip-iron-shafts/ I use them and play the "R" myself in a set of Swing Science Pro Forged and they do drop ball flight even soft stepping them 1x.
BTW, should add on average the Modus shafts trajectory lowest to highest is the 125 next 105 then 120 last 130.
I wouldn't play or fit a player a tip stiff shaft without a late release. Can't compress will "feel" like crap.