Start with something very "simple": force him to work lightly, without any force from the arms and shoulders. Insist on releasing the arms and shoulders (waving them etc.) before each technique application. Stop him each time he starts to use force instead of movement. When he does things correctly, flow with him - no resistence at all.
Once the concepts start to settle, start reducing the level of cooperation, Uke, instead of e moving as a response to every hint of direction, may start slowing down, making him, as Tori, create more power.
On a second thought - he should also practive being such an Uke - moving to any hint of power\direction, flowing with minor intent. That too should teach him to flow.
Both of these methods may seem "not realistic" or "non practical for fighting", unless you ever tried playing with someone who was soft enough for you not to feel him, and then felt how powerless this made you, being unable to create any force on him.