Hi.

Try some scap stabillisation with "core activation" through the T/S (multifidus).

If he is getting pain on initiation of movement, it is usually because there is not a stable position for the joint to transfer the load therefore the tendon/stressed structure is bearing the load.

Try also some eccentric bicep curls without the concentric part (i.e. get the other hand to help lift the weight up). Then gradually move the weight further away with gradually less elbow flexion until you have the eccentric version of bringing your arm dwon from FF. Focus particularly on the last few degrees where he gets his pain. The eccentric exercise should help his control and stability and allow him to load up more than he can with concentric exercise.

Let us know how he goes