But this is beginner stop and go aikido, so I tighten up the grip with a little elbow atemi to the back to prevent the twist. I gently suggest that twisting away from shihonage is a good way to get hurt, much safer to turn a little towards nage, easier on shoulder, better ukemi with more ukemi options.
Perhaps if you led him off balance more before you tried to go under the arm, he wouldn't be able to twist?
When I find my newbie partners can twist out it's not only because I allow them the room, but because I didn't take their balance before I entered, in deference to their newbie status. Easy correction.