Lots of core and internal rotation strengthening. No doubt he was strengthening the Lats, and also strengthening the core in a rotational/diagonal fashion very beneficial to throwing
Is your son tall?
In my empirical research I find that many taller ball players/pitchers specifically, have weaker Lats and core due to their shear body length…they have such long lever arms to create force, but the muscles themselves aren’t that strong.
Happy he’s feeling better. Best of luck!